What does irrational mean?
Irrational describes something that is not governed by reason, logic, or sound judgment. People use it for decisions, fears, beliefs, and reactions that seem excessive, confused, or impossible to justify. The term can also describe a failure to use evidence or think consistently, as in an irrational response to criticism. In mathematics, an irrational number is a real number that cannot be written as a ratio of two whole numbers; the square root of two is a standard example. The word is more forceful and judgmental than merely “unwise,” because it suggests that the relevant behavior lacks a rational foundation rather than simply proving unsuccessful. It is also useful in formal discussion because it identifies conduct or belief that conflicts with reason, while the technical mathematical sense is precise and clearly distinguished from everyday criticism.
adjectiveNot based on reason, logic, or sound judgment; irrational behavior or beliefs are unreasonable, confused, or contrary to accepted principles. In mathematics, the term describes numbers and expressions that are not ratios of whole numbers.
- Not based on reason, logic, or sound judgment; unreasonable or contrary to accepted principles.
- In mathematics, not expressible as a ratio of two whole numbers, or involving such a quantity.
"The decision to ignore the warning signs was irrational and led to unnecessary risk."
"Fear of unfamiliar places led him to make an irrational choice that everyone else regarded as absurd."
"The square root of two is an irrational number because it cannot be expressed as a fraction of two integers."
Irrational follows the mathematical idea of a number that cannot be expressed as a ratio of whole numbers.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does irrational come from?
The adjective comes from Latin irrationalis, meaning “not rational” or “without reason,” formed from the negative prefix in- and the related Latin word ratio. It entered English through Latin and French usage. Its basic sense developed from the general opposition between reason and unreason, while the mathematical sense follows the related concept of an irrational number, a quantity that cannot be expressed as a ratio of integers.
Related word forms
How irrational is actually used
In general use, the word is critical of a belief, decision, emotion, or action that lacks reason or sensible judgment. Its mathematical sense is technical and should not be confused with ordinary claims that are merely unreasonable.
Easily confused with irrational
An irrational belief or action lacks a logical basis, whereas a rational one is based on reason and sound judgment.