An idiom is defined as - a group of words whose meaning is different from the meanings of the individual words.
Idiom:
rule the roost
Meaning:
to be the dominant figure in a family
Example:
The woman seems rather quiet but she rules the roost in her family.
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.