rational

rational — adjective

1. involving decisions, beliefs, or actions that are based on facts and careful thi

1.adjectiveB1
Definition

involving decisions, beliefs, or actions that are based on facts and careful thinking rather than on feelings or personal wishes

Example

Lisa made a rational choice to leave early and avoid the traffic.

collocation: rational choice

It is not always rational to let fear guide your decisions.

impersonal: it is rational to do something

Common collocations
  • rational choice
  • rational decision
  • rational explanation
  • rational argument
  • rational approach
  • rational response
Synonyms
  • reasonable

    more general; can apply to prices, amounts, and expectations, not just decisions

  • logical

    emphasises step-by-step reasoning and consistency rather than practical good sense

  • sensible

    practical and showing good judgment, with less emphasis on formal reasoning

Antonyms
  • irrational

    the direct opposite; based on emotion rather than reason

  • emotional

    driven by feelings, the opposite of detached logical thought

  • unreasonable

    not fair or sensible; often used for demands or expectations

Grammar Patterns

rational + noun

be rational to + infinitive

rational about + noun phrase

Usage Note

Evaluates specific choices, decisions, or arguments rather than a person's general thinking capacity — compare sense 2 (REASONING CAPACITY). Frequently used in the pattern 'it + be + rational + to-infinitive'.

Common Mistakes

It was a rational decision to buy the expensive car without checking the price.
It was a rational decision to compare prices before buying the car.
A rational decision involves careful thought and facts; buying without checking is the opposite of rational.
She stayed rational after hearing the bad news.
She stayed calm after hearing the bad news.
'Rational' describes thought and decision-making; 'calm' describes emotional control in the moment.

2. having the ability to think clearly, understand ideas, and make judgments based

2.adjectiveB2
Definition

having the ability to think clearly, understand ideas, and make judgments based on logic rather than feelings or instincts

Example

Human beings are rational creatures who can plan for the future.

collocation: rational creatures

Theo was too upset to be rational about what had happened.

rational about + noun clause

Common collocations
  • rational being
  • rational creature
  • rational mind
  • rational thought
  • rational faculty
Synonyms
  • thinking

    broader; describes any cognitive activity, not necessarily logical

  • intellectual

    emphasises the use of intellect and knowledge, often in academic contexts

  • reasoning

    emphasises the process of drawing conclusions from evidence

Antonyms
  • irrational

    lacking the ability to think logically; driven by instinct or emotion

  • instinctive

    based on natural impulse rather than reasoned thought

Grammar Patterns

rational + noun (being, creature, mind)

be rational about + noun phrase

not rational + adverbial

Usage Note

Describes beings (usually human) who possess the general capacity to think and reason. Compare sense 1 (LOGICAL THINKING), which evaluates individual actions rather than the person's overall faculty. In legal and medical contexts, 'not rational' can refer to a temporary state where judgment is impaired.

Common Mistakes

Dogs are rational animals.
Dogs are intelligent animals, but they are not rational in the way humans are.
'Rational' as the capacity for abstract reasoning is typically reserved for humans.
She was too tired to make a rational thinking.
She was too tired to think rationally.
'Thinking' is already a noun; the phrase is 'rational thinking' without 'a', or use the adverb form 'rationally'.

3. describes any number you can write as a fraction made from two integers, like 1/

3.adjectiveB2
Definition

describes any number you can write as a fraction made from two integers, like 1/2, 3/4, or 7

Example

Every whole number is rational because it can be written as a fraction.

predicative use with because-clause

Ishaan learned to add rational numbers like one half and three quarters.

collocation: rational numbers

Common collocations
  • rational number
  • rational expression
  • rational function
Synonyms
  • fractional

    narrower; refers only to fractions, not whole numbers

  • arithmetic

    broader; describes any number used in basic calculations

Antonyms
  • irrational

    of a number that cannot be written as a fraction, such as π or √2

Grammar Patterns

rational + noun (number, expression)

Usage Note

Used only in mathematics. Never describes people or behavior. The opposite is irrational (a number that cannot be written as a simple fraction, such as π or √2).

Common Mistakes

The square root of four is an irrational number.
The square root of four is a rational number because it equals 2.
√4 = 2, which is a whole number and therefore rational.

rational — noun

Word family

  • rationalize
    verbB2
  • rationalization
    nounC1
  • rationality
    nounC1
  • rationally
    adverbB2
  • irrational
    adjectiveB2
  • rationalism
    nounC1
  • rationalist
    nounC1

Synonym discussion

Sense: adjective/1

All three words describe choices or arguments that are well-founded, but they highlight different qualities. 'Reasonable' is the broadest: it covers fair prices, acceptable requests, and moderate expectations — for example, a reasonable rent increase that stays close to market rates. 'Logical' focuses on whether the steps of an argument follow one another correctly, even if the starting point is unusual. A logical conclusion can be reached from bad premises, whereas a rational judgment typically starts from sound facts. 'Sensible' is the most practical word of the three: a sensible decision works well in real life, such as taking an umbrella when rain is forecast. In everyday use, 'rational' sits closest to 'reasonable' but adds a contrast with emotion — you remain rational even when you feel upset, while a reasonable outcome simply avoids being extreme.

reasonable ↗logical ↗sensible ↗

Common collocations

Nouns

  • rational behavior
  • rational thought
  • rational decision-making
  • rational mind

Adverbs

  • completely rational
  • entirely rational
  • perfectly rational
  • purely rational
  • strictly rational

Prepositions

  • rational about
  • rational in

Etymology

LatinFrenchEnglish

The word 'rational' comes from Latin 'rationalis', meaning 'of or belonging to reason'. This was formed from 'ratio', which originally meant 'calculation' or 'reckoning' and later broadened to 'reason' in a philosophical sense. The word entered English in the late 14th century through Old French 'rationel'. The mathematical sense — describing numbers that can be expressed as a ratio — developed in the 16th century, when mathematicians began using 'ratio' to describe the relationship between two quantities.