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.
Idiom:
loom large (on the horizon)
Meaning:
to be coming as a possible problem or danger or threat
Example:
A large increase in transportation costs is looming large on the horizon.
List of common idioms, their meanings and examples of their usage in sentences
Idioms can be a powerful linguistic tool only when they are used in the right way in the right places.