A conclusive or critical test, as used to determine the worth, quality, or effectiveness of someone or something. The phrase comes from the 19th-century practice of testing metals in nitric acid to determine if they contained gold. Restructuring the organization will be the acid test that determines whether or not it can survive the sudden downturn in the economy. The first game of the season will be an acid test of the team's head new coach. The acid test for the new manager will come in the form of healthcare open enrollment. We really need it to go smoothly this year.