An idiom is a group of words, or in other words, a phrase that has a meaning different from the literal meaning of the words in it.
Idiom:
like a sitting duck
Meaning:
unsuspecting and unaware
Example:
The robber was like a sitting duck when the police arrested him as he waited for his partner to leave the bank.
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.