Definition of acid test noun from the Oxford Advanced Learner's Dictionary

 

acid test

 noun
noun
BrE
 
; NAmE
 
(also litmus test especially in North American English)[singular]
 
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a way of deciding whether something is successful or true The acid test of a good driver is whether he or she remains calm in an emergency. The perception of fairness is the acid test for democracy. This type of stone is the acid test of a climber’s ability. Word Originfiguratively, from the original use which was a test for gold using nitric acid.
See the Oxford Advanced American Dictionary entry: acid test