An idiom is a widely used saying or expression containing a figurative meaning that differs from the phrase's literal meaning.
Idiom:
litmus test
Meaning:
a test that uses one single indicator to help make a decision (the scientific meaning of a litmus test is a test for basic chemical properties that uses litmus paper)
Example:
The new sales policy was a litmus test for how the new manager would manage the department.
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.