An idiom is a type of phrase or expression that has a meaning that can't be deciphered by defining the individual words.
Idiom:
ground (someone)
Meaning:
to take away someone's privileges (usually used for teenagers)
Example:
The girl was grounded by her parents because she came home late too often.
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.