sea

sea — noun

1. the salty liquid found in the planet's deepest basins, between the land masses,

1.nounA2
Definition

the salty liquid found in the planet's deepest basins, between the land masses, or a named stretch of this water partly enclosed by land and not as big as an ocean

Example

The children spent every afternoon swimming in the warm sea during their holiday in Greece.

the + sea for a specific body of water

The Mediterranean Sea connects Europe to Africa and is a major shipping route.

proper name + Sea for a named sea

Common collocations
  • the sea
  • by the sea
  • at sea
  • out to sea
  • sea level
  • sea water
  • deep sea
Synonyms
  • ocean

    larger than a sea; 'ocean' refers to one of the five major divisions of the world's salt water

  • marine

    adjective only; used to describe things relating to the sea

  • brine

    literary or scientific; refers to very salty water, not commonly used in everyday speech

Antonyms
  • land

    the solid part of Earth's surface as opposed to the sea

  • shore

    the edge of the land next to the sea

Grammar Patterns

the + sea

[proper name] + Sea

a/the + sea of + noun

Usage Note

Can be uncountable ('the sea is blue') or countable with a proper name ('the Black Sea'). When referring generally to the world's salt water, use 'the sea' without a capital letter. Named seas take the definite article and capital letters.

Common Mistakes

I like sea.
I like the sea.
'sea' is usually preceded by 'the' when referring to it as a general body of water.
The Sea of Japan is very deep.' (when referring to the body of water as 'Japan Sea')
The Sea of Japan is very deep.' (correct
some seas have 'Sea of' structure)

2. a very large number or amount of people or things that fills an area and is diff

2.nounB2
Definition

a very large number or amount of people or things that fills an area and is difficult to see the end of, like a wide stretch of water

Example

The singer walked on stage and saw a sea of waving hands and bright lights.

a sea of + plural noun for large number

After the storm, a sea of fallen leaves covered every path in the park.

Common collocations
  • a sea of
  • a sea of faces
  • a sea of people
  • a sea of light
Synonyms
  • ocean

    also used figuratively, e.g. 'an ocean of possibilities'; similar in meaning but slightly less common

  • flood

    suggests a rapid, overwhelming arrival rather than a broad expanse

Grammar Patterns

a sea of + plural noun

Usage Note

Almost always used in the fixed phrase 'a sea of + [plural noun]'. The noun that follows is typically countable and describes visible items or people. This is a figurative extension of the literal meaning.

Common Mistakes

There was sea of people.
There was a sea of people.
the article 'a' is required before 'sea' in this figurative expression.

3. the open ocean, especially as the destination or setting of a ship's journey aft

3.nounB1
Definition

the open ocean, especially as the destination or setting of a ship's journey after leaving port, or the state of travelling on a ship away from land

Example

The captain ordered the crew to put to sea as soon as the storm passed.

put to sea: leave port by ship

Nikos spent six months at sea on a cargo ship travelling between Asian ports.

at sea: on a voyage away from land

Common collocations
  • at sea
  • put to sea
  • put out to sea
  • go to sea
  • lost at sea
Synonyms
  • voyage

    a noun for the journey itself, not the location

  • open water

    describes the sea away from the coast, used in both literal and technical contexts

Grammar Patterns

at sea

put to sea

go to sea

be lost at sea

Usage Note

This meaning appears almost exclusively in fixed prepositional phrases. 'At sea' describes being on a voyage; 'put to sea' describes the act of leaving port; 'go to sea' describes starting a career or voyage as a sailor. The literal noun 'sea' in these phrases refers to the open ocean as a space for travel.

Common Mistakes

The ship is in the sea.' (when meaning it is sailing)
The ship is at sea.
'at sea' is the correct fixed phrase for a ship that has left port.

4. any of the dark, level patches found on the lunar surface which early observers

4.nounC1
Definition

any of the dark, level patches found on the lunar surface which early observers wrongly believed to be oceans but are in fact dry volcanic plains

Example

The Apollo 11 crew landed near the edge of the Sea of Tranquillity on the moon.

proper name: Sea of + Latin name

Through his telescope, Felix saw the dark shapes called seas on the moon.

Common collocations
  • Sea of Tranquillity
  • lunar sea
  • mare (pl. maria)
  • moon's seas
Synonyms
  • mare

    the Latin term used in astronomy; plural is 'maria'

  • lunar plain

    a more scientifically accurate description than 'sea'

Antonyms
  • highland

    the lighter, higher areas on the moon's surface, called 'terrae'

Grammar Patterns

the Sea of + [Latin name]

the + [Latin name] + Sea

Usage Note

Used only in technical astronomical contexts, almost always with a Latin proper name like 'Mare Tranquillitatis' or its English translation 'Sea of Tranquillity'. The singular Latin term is 'mare' and the plural is 'maria'.

Common Mistakes

The moon has real seas with water.
The moon has dark areas called seas, but they are actually dry lava plains.
a common misunderstanding for learners.

5. the condition of the surface of an ocean or lake in terms of wave size and movem

5.nounB1
Definition

the condition of the surface of an ocean or lake in terms of wave size and movement, especially when waves are large and dangerous

Example

The weather forecast warned of rough seas and strong winds along the entire coast.

rough seas: large, dangerous waves

A heavy sea swept across the deck and soaked the crew.

heavy sea: large, powerful wave(s)

Common collocations
  • rough sea
  • heavy sea
  • calm sea
  • sea conditions
  • dangerous seas
Synonyms
  • swell

    refers specifically to long, unbroken waves, not the overall condition of the surface

  • wave

    a single rising movement of water, not the overall condition; 'sea' describes the general state

Antonyms
  • calm

    the absence of large waves or rough movement

Grammar Patterns

rough/heavy/calm + sea(s)

sea conditions

Usage Note

In this sense 'sea' is often used in the plural ('the seas are rough') or with descriptive adjectives ('a heavy sea', 'a calm sea'). 'Rough sea' and 'heavy sea' are fixed collocations used in weather reports and maritime communication.

Common Mistakes

The sea is very heavy today.' (when describing wave conditions at a specific location)
The seas are very heavy today.
the plural form is more natural when describing wave conditions.
Idioms

at sea / all at sea

idiom

confused or uncertain about what to do, as if lost without direction

When the teacher changed the exam format at the last minute, most of the students were completely at sea.

Hana was all at sea during her first week at the new company because nobody explained the office rules to her.

sea change

idiom

a complete and dramatic transformation in someone's character, situation, or opinion

There has been a sea change in public attitudes towards renewable energy over the past decade.

Moving to a small village brought about a sea change in Mira's outlook on life.

a sea of

idiom

a very large number or amount of people or things filling an area

The stadium was filled with a sea of supporters waving flags and cheering for their team.

From the balcony, Chiara looked down at a sea of colourful umbrellas moving through the rainy streets.

between the devil and the deep blue sea

idiom

in a difficult situation where both available choices are equally unpleasant or harmful

Ife was between the devil and the deep blue sea: she could either lose her job or work with a manager she did not trust.

The company found itself between the devil and the deep blue sea when it had to raise prices or cut staff.

at sea

idiom

On the ocean, either traveling by ship or floating in the water.

The sailors spent three months at sea without seeing land.

After the storm, the small boat was lost at sea.

go/run away to sea

idiom

To leave home and become a sailor, often without telling anyone.

At sixteen, Tom ran away to sea and never returned.

She dreamed of going away to sea on a tall ship.

out to sea

idiom

Far from the coast, where the water is deep.

The fisherman rowed his boat far out to sea.

A strong current carried the swimmer out to sea.

put (out) to sea

idiom

To begin a journey by ship, leaving the port or harbor.

The captain ordered the crew to put to sea at dawn.

Despite the fog, the ferry put out to sea on time.

there are plenty more fish in the sea

idiom

Used to tell someone whose romantic relationship has ended that they will find another partner.

After her breakup, Maya's friend said, "There are plenty more fish in the sea."

Don't worry about losing him; there are plenty more fish in the sea.

Word family

  • seaside
    nounA2
  • seafood
    nounA2
  • seashell
    nounB1
  • seashore
    nounB1
  • seasick
    adjectiveB1
  • seasickness
    nounB1
  • seafront
    nounB2
  • overseas
    adverbB1
  • seafaring
    adjectiveC1
  • seascape
    nounC1
  • seagoing
    adjectiveC2

Synonym discussion

Sense: noun/1

While 'sea' and 'ocean' both refer to large bodies of salt water, an ocean is significantly larger and there are only five on Earth (Pacific, Atlantic, Indian, Southern, Arctic). Seas are typically smaller, partly enclosed by land, and are often part of an ocean — for example, the Mediterranean Sea is connected to the Atlantic Ocean. 'Marine' is an adjective, not a noun, and describes anything related to the sea ('marine biology', 'marine life'). 'Brine' is a technical or literary term for very salty water, used in chemical contexts or poetry rather than everyday speech. 'Waters' (plural) is a formal or literary alternative that can refer to a specific sea area ('the coastal waters of Japan'). In everyday use, 'the sea' is the most natural choice when referring to the vast body of salt water covering the planet.

ocean ↗marine ↗brinewaters

Common collocations

Nouns

  • sea water
  • sea breeze
  • sea level
  • sea bed
  • sea floor
  • sea air
  • sea life
  • sea salt
  • sea route

Adverbs

  • far out to sea
  • far out at sea

Phrases

  • the middle of the sea
  • the bottom of the sea
  • the edge of the sea
  • the open sea

Prepositions

Etymology

Proto-Indo-EuropeanProto-GermanicOld EnglishEnglish

The word 'sea' comes from Old English 'sǣ', meaning a large body of water or lake. It traces back to Proto-Germanic '*saiwiz', which also gave rise to Old Saxon 'sēo', Old Norse 'sær', Dutch 'zee', and German 'See' (meaning lake) and 'Meer' (meaning sea). The ultimate origin is uncertain, but some linguists connect it to a Proto-Indo-European root '*sh₂ey-wo-' meaning 'to be fierce' or 'to rage' — perhaps referring to the wild, untamed nature of the sea. The word has been used in English for more than a thousand years, remaining remarkably stable in its core meaning.