Acid test – A conclusive test of the success or value of something

Note: The expression comes from nitric acid that was used to test whether or not prospectors had legitimate gold. Rub the gold or metal onto a black stone to leave a mark. Apply nitric acid. If the mark remains, it's gold, and the mark of any other metal found in a mine will dissolve. 

To remember the definition of the word acid test, use the following mnemonic:

The acid was tested (acid test) on the metal. It didn't dissolve, so it was a conclusive test of success. 

Examples of acid test in a sentence

The police chief's true acid test would be his ability to resist corruption. 

Adversity is often the acid test of friendship. 

The show failed the network's acid test and was cancelled as a result.