litmus

In a chemical context, this term refers specifically to the dye derived from lichens. It is almost exclusively used as an attributive noun to modify other words, such as in "litmus paper" or "litmus solution," rather than standing alone as a mass noun.

When used figuratively, it appears almost exclusively in the compound phrase "litmus test." This expression denotes a single, decisive criterion that determines whether something is acceptable or true. It carries a connotation of binary results—pass or fail, yes or no—leaving little room for nuance or middle ground.