key
key — noun
1. a small metal object cut into a particular shape that you put into a lock to ope
a small metal object cut into a particular shape that you put into a lock to open or close a door, start a vehicle, or operate something that needs to be secured.
Élise put the house key into her bag before leaving for school.
house key — type of key by function
Ryo realised he had left his car keys on the kitchen table.
car keys — plural for set of keys
The old iron key to the garden gate was rusted and hard to turn.
Beatrix handed a spare key to her neighbour so someone could water the plants.
spare key — extra copy of a key
- spare key
- car key
- house key
- master key
- key ring
- key chain
Grammar Patterns
key + to/of + noun phrase
Usage Note
Countable noun. When referring to a set that belongs to one person, the plural form (my keys) is very common even if the speaker owns only two or three.
Common Mistakes
2. one of the small raised parts found on keyboards and musical instruments that us
one of the small raised parts found on keyboards and musical instruments that users push down to type text, enter commands, or play notes.
Gabriel pressed the wrong key and the whole document disappeared from the screen.
press a key — common verb collocation
The black keys on the piano are set in groups of two and three.
Christopher cleaned the sticky keys on his laptop keyboard with a small brush.
Some typewriter keys would jam together if you typed too quickly.
typewriter keys — mechanical keyboard keys
- press a key
- hit a key
- keyboard key
- piano key
- function key
- arrow key
Grammar Patterns
key + on + noun phrase
Usage Note
On a computer, individual keyboard buttons are called keys, whereas on a touchscreen the corresponding areas are called buttons or icons.
3. a group of notes built around a central note that gives a song or instrumental w
a group of notes built around a central note that gives a song or instrumental work its overall character and determines which notes are most commonly used in it.
Tanvi asked the guitarist to play the song in a lower key so she could sing along.
in the key of — standard preposition pattern
The piece begins in C minor but shifts to a brighter key later on.
Chidi could not tell what key the trumpet melody was written in.
A major and A minor are closely related keys that often appear in the same piece.
- in the key of
- change key
- major key
- minor key
- key signature
- tonality
a more technical, formal term used in music theory for the same concept
Grammar Patterns
in the key of + musical note
Usage Note
Often used in the pattern 'in the key of + note name' (e.g., in the key of G major). The word 'key' also refers to the sharps or flats shown at the beginning of a musical staff (key signature).
4. a small chart or box on a map, diagram, or book that explains what the symbols,
a small chart or box on a map, diagram, or book that explains what the symbols, colours, or abbreviations used in it mean.
Noor checked the map key to find out what the dotted line across the forest represented.
map key — specific collocation
Without the key it was impossible to tell which symbols stood for hospitals and which for schools.
The diagram key showed that blue circles marked ancient ruins and red triangles marked lookout points.
Gabriel studied the key on the hiking map before choosing a trail for the afternoon.
- map key
- diagram key
- symbol key
- legend
a slightly more formal term, especially in cartography and technical diagrams
Grammar Patterns
the key to + noun phrase
Usage Note
Also called a 'legend'. 'Key' is more common in everyday language; 'legend' is slightly more formal and is the preferred term in cartography.
Common Mistakes
5. a list or sheet that shows the correct answers for the questions in a test, exer
a list or sheet that shows the correct answers for the questions in a test, exercise, or puzzle, used by teachers or by learners to check their own work.
The teacher kept the answer key locked in her desk drawer during the exam.
answer key — standard collocation
Heloísa checked her answers against the key at the back of the workbook.
Cyrus finished the crossword and looked for the key to see if his answers were correct.
Karim found a typo in the answer key and told the teacher about it.
- answer key
- marking key
- key to the exercises
- answer sheet
a physical sheet where answers are written, not the list of correct answers
Grammar Patterns
answer key
key to + noun phrase
Usage Note
Almost always used in the phrase 'answer key'. Learners often confuse 'key' (the answer list) with the test itself.
Common Mistakes
6. a small, low-lying island, usually in warm ocean waters, formed from coral sand
a small, low-lying island, usually in warm ocean waters, formed from coral sand or rock, and often shaped like a ring.
The boat sailed past a tiny key covered only with palm trees and white sand.
tiny key — descriptive of small island
Anong spent the afternoon snorkelling in the clear water around a coral key off the coast.
Many seabirds nest on the rocky keys that lie scattered across the bay.
The ferry stops at several small keys before reaching the main island.
- coral key
- Florida Keys
- small key
- tropical key
- cay
an alternative spelling, pronounced the same way, more common in the Caribbean
Usage Note
This sense is mostly used in place names in the Caribbean and Florida (e.g., the Florida Keys). Outside those regions the word 'cay' (pronounced the same way) is more common.
7. the rectangular painted area on a basketball court near each basket, where certa
the rectangular painted area on a basketball court near each basket, where certain rules about player positioning and fouls apply.
Karim drove towards the basket and stepped into the key before making a jump shot.
step into the key — common basketball move
The referee called a three-second violation because the centre had stayed in the key for too long.
Players cannot stand inside the key for more than three seconds while their team has the ball.
Noor passed the ball to a teammate at the top of the key for an open shot.
top of the key — basketball position
- in the key
- the painted key
- three-second key
- top of the key
- free-throw lane
the official term in basketball rulebooks; 'key' is the everyday player term
- the paint
an informal term based on the fact that this area is usually painted a different colour
Usage Note
Also called the 'free-throw lane' or 'the paint'. In official basketball rules, this is the area between the free-throw line and the baseline, bounded by lane lines.
8. something that provides a way to understand, achieve, or gain access to somethin
something that provides a way to understand, achieve, or gain access to something else, often used figuratively to describe the most important factor for success or comprehension.
Hard work and patience are the key to mastering any new skill.
the key to + gerund — common pattern
The librarian said that regular practice was the key to improving your reading speed.
A good night's sleep could be the key to solving difficult problems more easily.
The police believed the missing phone was the key to the mystery.
key to the mystery — figurative use
- key to success
- key to understanding
- key to the mystery
- key to happiness
- barrier
something that prevents access or understanding
Grammar Patterns
the key to + noun phrase / gerund
Usage Note
Always followed by 'to' (not 'of') when indicating what is being achieved or understood: the key to success, the key to the puzzle. The sense is singular even when paired with a plural subject: 'Hard work and discipline are the key to…' (treating 'key' as a singular concept).
Common Mistakes
Something is stored in a locked place to keep it safe, or a person is kept in prison.
Maya keeps her grandmother's jewelry under lock and key.
The judge said the thief must stay under lock and key.
key — adjective
1. extremely important or essential, especially because other things depend on it o
extremely important or essential, especially because other things depend on it or are strongly affected by it.
Communication is a key skill for anyone working in a team environment.
key skill — common attributive use
The finance director played a key role in the company's decision to expand overseas.
key role — common collocation
Sunlight is a key factor in helping young plants grow strong and healthy.
Christopher was a key player in the negotiations between the two sides.
- key factor
- key role
- key player
- key element
- key point
- key issue
- key decision
- minor
of little importance or influence
- unimportant
not having any significant effect
Grammar Patterns
key + noun
key to + noun phrase / gerund
Usage Note
Used only before a noun (attributive position). You cannot say 'This factor is key' in formal written English, though you may hear it informally. In formal contexts, use 'crucial' or 'essential' predicatively: 'This factor is crucial.'
Common Mistakes
key — verb
1. to deliberately scratch the surface of a car or other vehicle with a metal key,
to deliberately scratch the surface of a car or other vehicle with a metal key, usually to damage or vandalise it.
Someone keyed the side of Ryo's new car while it was parked outside the cinema.
key + someone's car — typical object pattern
The security camera caught a person keying vehicles in the parking lot after midnight.
keying vehicles — gerund form
Karim was furious when he found a long scratch where someone had keyed his door.
The neighbour admitted to keying the truck after an argument about the parking space.
- key a car
- key someone's vehicle
- get keyed
- scratch
more general term; does not specify the tool used
Grammar Patterns
key + noun phrase
Usage Note
This sense is almost always used with a car as the direct object. The action is considered vandalism and is illegal in most places. The word is informal but widely understood.
Common Mistakes
key — idiom
1. Francis Scott Key (1779–1843), an American lawyer and poet who wrote the words t
Francis Scott Key (1779–1843), an American lawyer and poet who wrote the words to 'The Star-Spangled Banner', the national anthem of the United States.
Francis Scott Key wrote the poem during the War of 1812.
biographical name — historical figure
A statue of Francis Scott Key stands in a park near Baltimore harbour.
Schoolchildren across America learn about Francis Scott Key and his famous song.
The bridge named after Francis Scott Key collapsed in 2024 after a ship struck it.
Usage Note
This is a biographical name, not an idiom. It appears here because the source grouped it as a multi-word expression. The name 'Key' in this sense is always a proper noun and refers specifically to the historical figure.
2. Sir John Phillip Key (born 1961), a New Zealand politician who served as the 38t
Sir John Phillip Key (born 1961), a New Zealand politician who served as the 38th Prime Minister of New Zealand from 2008 to 2016.
John Key led the National Party to three election wins.
biographical name — NZ prime minister
During his time in office, John Key focused on economic recovery after the 2008 crisis.
Many New Zealanders remember John Key for his response to the Christchurch earthquake.
John Key resigned as Prime Minister in December 2016 after eight years leading the country.
Usage Note
This is a biographical entry for the former prime minister. The surname 'Key' here is unrelated to the other senses of the word and is a family name.
Word family
Synonym discussion
Sense: adjective/1
All four words describe something of great significance, but they differ in strength and context. 'Key' is the most common choice in everyday business and academic settings (a key factor, a key role) and is used only before a noun. 'Crucial' carries a slightly stronger sense of a decisive moment or turning point (a crucial experiment, a crucial decision). 'Essential' emphasises that something is absolutely necessary and cannot be done without (water is essential for life). 'Vital' is the strongest of the group, suggesting that without it something would cease to function or die (vital organs, vital information). In practice: 'Regular practice is key to progress' (common idiom), but 'Clean water is essential for health' (necessary condition), and 'This is a crucial step in the process' (decisive moment).
Common collocations
Phrases
- key to success
- key to understanding
- key role in
- key factor in
Prepositions
Etymology
The word 'key' comes from Old English 'cǣg', spelled 'cæg' in early records, meaning 'metal device for opening a lock'. Its origins before Old English are unknown — it has no clear relatives in other Germanic languages, which suggests it may have been a very early borrowing from a non-Germanic source. The figurative sense ('a means of access or understanding') developed in Middle English, following the natural metaphor of a key unlocking not just doors but also knowledge. The musical sense appeared in the 15th century, describing how one note 'unlocks' the melody. The geographical sense (small island) comes from Spanish 'cayo' (shoal, small island), influenced by the similarity in sound and spelling to 'key'.