An idiom is a type of phrase or expression that has a meaning that can't be deciphered by defining the individual words.
Idiom:
tone (something) down or tone down (something)
Meaning:
to make something less harsh or strong, to moderate something
Example:
The union leader was forced to tone down his language after the strike became violent.
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.