An idiom is a type of phrase or expression that has a meaning that can't be deciphered by defining the individual words.
to shake hands in a friendly way
The politician likes to glad hand people at the shopping center.
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.