An idiom is defined as - a group of words whose meaning is different from the meanings of the individual words.
Idiom:
lost in thought
Meaning:
to be busy thinking
Example:
I was lost in thought when my friend phoned last night.
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.