litmus.

1. A dye from lichens that changes color according to acidity or alkalinity.

2. A decisive indication of a condition or commitment, especially in litmus test.

Use it for

Words that belong together

• litmus paper
• litmus test

On the page

The solution turned blue litmus paper red.

Usage notes

Primary category: analysis / argumentation / evidence. Forms: Usually uncountable; no ordinary plural in this sense. Legacy lexical fields mirror sense 1 only. Use numbered sense fields for all meanings. Established modern English lexical item. No separate regional spelling or grammatical inflection entry. Writing functions: argumentation, evidence, inference. Commonness: moderately common.