What does the idiom "by definition" mean?

The expression by definition is one of the idioms that often finds a place in our literature and enriches our language. However, its meaning is not fully understood, so it is sometimes used in the wrong situations. Please review the explanation carefully for the correct use of the by definition idiom.

Meaning of "by definition"


The phrase “by definition” is generally used to refer to something that is exactly as it is described, according to its dictionary definition or established set of rules. It implies that the thing being discussed is unambiguous and there is no room for misunderstanding. The phrase is often used to check that both parties in a conversation clearly understand each other.


The phrase “by definition” has been used in the English language since at least the mid-19th century. It comes from the Latin phrase “de definitio” which translates as “of defining”. This phrase is thought to have been used to refer to a definition of a term or phrase and was used to express certainty in understanding.


The phrase “by definition” is used to make a statement more precise. For example, it can be used to show that something is definitively true, or that someone understands the exact definition of something. It can also be used to check that two people are discussing the same thing, or to say that a statement is true beyond a reasonable doubt.

Example Sentences

  • I know what the word means ‘by definition’.
  • By definition, a square is a four-sided shape with all sides equal.
  • This must be done correctly ‘by definition’.

The meanings of the words in the "by definition" idiom

The Surprising Origins of Everyday English Idioms

Many English idioms have surprisingly dark origins, often rooted in violence, death, and superstition. For instance, the phrase "raining cats and dogs" is said to have originated in the 17th century, when heavy rain would often cause dead animals to wash up on the streets. Meanwhile, the idiom "rule of thumb" is believed to have originated from a law that allowed men to beat their wives with a stick no thicker than their thumb.


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