- a test that proves a hypothesis or validity of a product or idea
- a test which really proves the value, quality or truth of something
- a sure or decisive test
- a test whose results are beyond doubt or dispute
- a test that is foolproof and conclusive
- He played well in the last match, but the acid test will come when the team plays the stronger opponents.
- The acid test for the new theory would come when it is actually implemented and the results come out.
- The politician made a lot of promises, but the acid test would come when he assumes office and starts delivering on them.
- He's had an easy tournament so far, but the next match would be his first real acid test.
- The acid test for the budding singer was when he had his first big show and had to sing in front of a large audience.
- The product look great, but will people buy it? That's the acid test.
The phrase alludes to a chemical test to prove the pureness of gold. This test was developed in the late 18th century and used nitric acid, which dissolve other metals more readily than gold. So the amount of metal dissolved would prove the pureness of the gold. The figurative use started around the mid 19th century, with the earliest print reference being in the Wisconsin paper "The Columbia Reporter" in November 1845.