rat

rat — noun

1. A small furry animal with a pointed nose, a hairless tail, and strong front teet

1.nounA2
Definition

A small furry animal with a pointed nose, a hairless tail, and strong front teeth. These animals grow bigger than common mice and often live in or near human homes, where they may be treated as pests or kept as pets.

Example

Hana saw a large rat run across the kitchen floor and screamed.

Ritu keeps two pet rats in a glass cage with wood shavings and a wheel.

collocation: pet rat

Common collocations
  • pet rat
  • lab rat
  • rat problem
  • rat infestation
  • rat poison
  • rat trap
Synonyms
  • rodent

    much broader category that includes mice, squirrels, and beavers

  • vermin

    negative connotation; refers to any harmful pest animal, not just rats

Common Mistakes

I saw a mouse in the sewer — it was huge.
I saw a rat in the sewer
it was huge.' — Use 'rat' for the larger rodent with a thicker tail; use 'mouse' for the smaller one with a thinner tail.
The lab mouse was used for the drug experiment.' (when referring to Rattus norvegicus).
The lab rat was used for the drug experiment.
Lab rats belong to a different species than lab mice; researchers distinguish them.

2. An insulting word for someone who cheats, lies to, or leaves behind the people w

2.nounB2
Definition

An insulting word for someone who cheats, lies to, or leaves behind the people who trusted them, such as friends or work partners.

Example

Bilal called his former business partner a dirty rat after losing all his savings.

collocation: dirty rat

Élise realized a friend had turned rat when the police arrived at her door.

phrasal: turn rat

Common collocations
  • dirty rat
  • lying rat
  • turn rat
  • rat fink
Synonyms
  • traitor

    more general; can refer to betraying a country, group, or cause

  • informer

    neutral or negative; specifically refers to someone who gives information to authorities

  • backstabber

    suggests betrayal through secret or dishonest actions

  • snitch

    slang; specifically refers to someone who tells on others, especially to police

Antonyms
  • ally

    someone who supports and stands by you

  • loyal friend

    someone who stays faithful even in difficult situations

Usage Note

Strongly offensive. Avoid using in polite or professional conversation. Often appears in fixed name-calling phrases such as 'dirty rat' or 'lying rat.'

3. A fast medical test that checks for a specific virus or bacteria by looking for

3.nounC1
Definition

A fast medical test that checks for a specific virus or bacteria by looking for proteins called antigens on a small sample, usually taken from the nose or throat.

Example

The school required a negative RAT result before students could return to class.

abbreviation: RAT stands for rapid antigen test

Anong took a RAT at home and was relieved to see only one line appear.

Common collocations
  • RAT result
  • RAT kit
  • take a RAT
  • negative RAT
  • positive RAT
Synonyms
  • rapid test

    shorter form, less specific about the type of test

Antonyms
  • PCR test

    a different type of test that looks for genetic material and takes longer to process

Usage Note

Almost always written in all-caps (RAT) when used as an abbreviation. Spoken as separate letters (R-A-T), not as the word 'rat.' Common in public health and medical contexts, especially during the COVID-19 pandemic.

4. A small device with spinning blades that drops out of an aircraft when the main

4.nounC2
Definition

A small device with spinning blades that drops out of an aircraft when the main engines fail, using the flow of outside air to produce emergency electricity or power for the control systems.

Example

When both engines shut down, the RAT deployed automatically beneath the aircraft's wing.

passive: RAT deployed

The pilot checked that the RAT had enough hydraulic pressure to move the control surfaces.

Common collocations
  • RAT deploys
  • RAT provides power
  • RAT actuator
  • hydraulic RAT

Usage Note

Almost always written as the abbreviation RAT in aviation contexts. Spoken as 'R-A-T,' not as 'rat.' This term is extremely specialized and appears almost exclusively in aircraft maintenance manuals and pilot training materials.

Idioms

smell a rat

idiom

to begin to feel that something is not honest, correct, or safe, especially when others appear to be hiding the truth

When the car salesman offered to cut the price by half, Nellie's father smelled a rat and walked away.

The accountant smelled a rat when she noticed that several payments had no receipts attached.

(like rats) deserting/leaving a sinking ship

idiom

This idiom describes people who leave a failing organization without concern for those who remain.

When the company announced layoffs, several managers were like rats leaving a sinking ship.

Investors, like rats deserting a sinking ship, sold their shares as the scandal grew.

rat — verb

Word family

  • ratty
    adjectiveC1
  • ratter
    nounC2
  • ratting
    nounC2

Synonym discussion

Sense: noun/1

'Rat' and 'mouse' are both rodents, but rats are noticeably larger, with thicker tails and blunter noses. A 'mouse' is smaller, has a thinner tail, and is often seen as less threatening. 'Rodent' is the broad scientific category that includes rats, mice, squirrels, hamsters, and beavers — it is a neutral term you would use in biology or pet-store contexts. 'Vermin' is a negative label for any small wild animal that damages property or spreads disease, including rats, cockroaches, and pigeons. For example: 'The lab uses both rats and mice for different experiments' (neutral factual statement) versus 'The old barn was overrun with vermin like rats and mice' (implies disgust and a need for extermination). Use 'rodent' when you want to be precise and scientific; use 'rat' when you want to name the specific animal; and use 'vermin' only when you want to express a strong negative attitude toward pests.

rat ↗mouse ↗rodentvermin

Common collocations

Nouns

  • rat poison
  • rat trap
  • rat race
  • rat infestation
  • rat problem
  • rat population

Phrases

  • like a rat in a cage
  • like rats leaving a sinking ship
  • smell a rat

Etymology

Proto-GermanicOld EnglishMiddle EnglishEnglish

The word 'rat' comes from Old English 'ræt,' which is related to similar words in other Germanic languages such as German 'Ratte' and Dutch 'rat.' These likely come from a very old root meaning 'to gnaw,' referring to the animal's habit of chewing through things. The use of 'rat' to mean a person who betrays others appeared in the early 1900s, probably from the idea of a rat abandoning a sinking ship. The medical abbreviation RAT (rapid antigen test) and the aviation term RAT (ram air turbine) are much more recent inventions from the 2000s and 1950s respectively, formed independently as acronyms.