An idiom is a type of phrase or expression that has a meaning that can't be deciphered by defining the individual words.
Idiom:
look up (someone) or look (someone) up
Meaning:
to seek and find someone
Example:
When I was in New York City I looked up my friend from university.
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.