Litmus test

Etymology

From chemistry, where litmus — a dye from lichens, its name via Old Norse for “dye-moss” — turns red in acid and blue in base, giving a fast pass/fail reading of acidity. The metaphor extended to any test that yields a simple, definitive verdict, as in a political “litmus test” issue.

noun phrase · /LIT-mus test/
A single decisive factor used to judge or classify something quickly.
What it names
The one clean criterion that sorts cases into camps, whether or not it deserves that much weight.
Example
For some voters, a candidate’s stance on that one issue is a litmus test that overrides everything else.
Related idea
A litmus test is a single decisive criterion; a benchmark is a standard you measure degrees of performance against, and a shibboleth sorts insiders from outsiders by a telltale marker rather than by merit.
noun · /LIT-mus test/
In chemistry, a test using litmus dye — paper that turns red in acid and blue in alkali — to tell whether a solution is acidic or basic.
What it names
the chemical acid-base indicator test
Example
Dip the strip in the liquid: if the litmus paper turns red, it’s acidic.
Related idea
Like the third rail, its figurative use borrows a vivid physical original — here, a single dip that gives one clear verdict.