An idiom is a type of phrase or expression that has a meaning that can't be deciphered by defining the individual words.
Idiom:
sucker list
Meaning:
a list of people who can be easily persuaded to buy something
Example:
The salesman used a sucker list to try and sell his new product.
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.