An idiom is a type of phrase or expression that has a meaning that can't be deciphered by defining the individual words.
Idiom:
thumb a lift
Meaning:
to hitchhike
Example:
Our car had a flat tire so I thumbed a lift to the nearest gas station.
List of common idioms, their meanings and examples of their usage in sentences
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.