Meaning of “the acid test” in the English Dictionary

british dictionary

"acid test" in American English

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acid testnoun [ C usually sing ]

us /ˈæs·ɪd ˈtest/

the proof of the value of something, or the standard that something must meet in order to prove its value:

In 1933, the pressurized airplane passed the acid test when it flew from Chicago to Washington at 22,000 feet above heavy storm clouds.

(Definition of “acid test” from the Cambridge Academic Content Dictionary © Cambridge University Press)