Litmus Test
A simple, single-issue test used to classify something — borrowed from the lab strip that turns red in acid and blue in base.
Origin
Litmus is a dye extracted from lichens (the name is Old Norse, 'colored moss'), used in chemistry since the 14th century. A strip of litmus paper dipped in solution turns red for acid, blue for base — a quick pH check that does not tell you how acidic or basic, only which side of neutral. The metaphor took off in American political writing in the mid-20th century, especially around Supreme Court nominations.
Modern usage
Standard political and managerial vocabulary. 'Abortion is a litmus test for that judge.' 'How they handle that interview is my litmus test.' Used to mean a single yes/no question that proxies for a larger judgment — sometimes a useful filter, sometimes a lazy substitute for thinking.
In the wild
His take on remote work is the litmus test for whether I'd take the job.— common usage
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