An expression in the usage of a language that is peculiar to itself either in having a meaning that cannot be derived from the conjoined meanings of its elements or in its grammatically atypical use of words.
Idiom:
go on a diet
Meaning:
to start a plan or program to decrease some foods in order to decrease one's weight
Example:
I plan to go on a diet at the beginning of the year.
List of common idioms, their meanings and examples of their usage in sentences
An idiom is a type of phrase or expression that has a meaning that can't be deciphered by defining the individual words.