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:
read (someone) like an open book
Meaning:
to understand someone very well
Example:
The girl can read her boyfriend like an open book.
List of common idioms, their meanings and examples of their usage in sentences
The language peculiar to a people or to a district, community, or class