An idiom is defined as - a group of words whose meaning is different from the meanings of the individual words.
Idiom:
in a word
Meaning:
briefly, to sum up
Example:
In a word, the problem with the car is that it needs a new motor.
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.