A litmus test is something (such as an opinion about a political or moral issue) that is used to make a judgment about whether someone or something is acceptable.

// At our family’s Thanksgiving dinner, the litmus test for good mac and cheese is whether or not it is baked.

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“The audience in a Broadway show can be intoxicating, and it’s like a litmus test. If a joke doesn’t land one night, you tell it differently the next night. It’s terrifying, on set, to have no idea if something is working.” — Erika Henningsen, quoted in The Hollywood Reporter, 1 May 2025

It was in the 14th century that scientists discovered that litmus, a mixture of colored organic compounds obtained from lichen, turns red in acid solutions and blue in alkaline solutions and, thus, can be used as an acid-base indicator. Six centuries later, people began using litmus test figuratively. It can now refer to any single factor that establishes the true character of something or causes something to be assigned to one category or another. Often it refers to something (such as an opinion about a political or moral issue) that can be used to make a judgment about whether someone or something is acceptable or not.