A litmus test is a test in which a single factor is decisive — one attitude, event, or fact used to sort something into one category or the other, and often used to judge whether a person or position is acceptable at all.
The figurative sense now dominates, but it borrows its precision from the chemistry. Litmus is a dye drawn from lichens that turns red in acid solution and blue in alkaline; paper soaked in it has served as an acid-base indicator since the fourteenth century. What the metaphor takes is not the accuracy of the test but its shape: litmus paper returns one of two colors, immediately, with no gradation and no partial credit. A litmus test answers yes or no, never how much.
The figurative use is twentieth-century American. One of the earliest citations comes from a 1940 New York Times column about college football, and the phrase spread most durably through politics, where a litmus-test issue is a single position — on a court nomination, a budget line, a social question — that determines endorsement or appointment regardless of everything else about the candidate. Loose usage has since stretched it to mean any consequential test, which blunts the image: a genuine litmus test is single-factor and instantaneous, not merely important.
That shape is also the word's built-in critique. A litmus test is the opposite of 📝discernment — it substitutes one legible index for judgment across many factors. The substitution is exactly why the test is fast, and exactly why it misreads anything complicated.