litmus

noun

lit·​mus ˈlit-məs 

1

: a coloring matter from lichens that turns red in acid solutions and blue in alkaline solutions and is used as an acid-base indicator

2

: the critical factor in a litmus test

also : litmus test

Examples of litmus in a Sentence

Recent Examples on the Web

In a month jam-packed with elections, the unique race serves as yet another litmus of Trump's endorsement in the midterms. —Nicole Fallert, USA Today, 26 May 2026

But if President Xi does make a move, whether by mounting a full-scale attack or using his military to blockade the island, the key question will be where such actions fall on the litmus scale. —Ned Temko, Christian Science Monitor, 12 Sep. 2025

And, of course, this is the first major litmus tense of the young and divisive Donald Trump presidency. —The Editorial Board, Chicago Tribune, 31 Mar. 2025

Her litmus to make sure these moments were successful? —William Earl, Variety, 23 Jan. 2025

Word History

Etymology

Middle English litmose, of Scandinavian origin; akin to Old Norse litmosi herbs used in dyeing, from litr color (akin to Old English wlite brightness, appearance) + mosi moss; akin to Old English mōs moss

First Known Use

14th century, in the meaning defined at sense 1

Phrases Containing litmus

• litmus paper
• litmus test

Medical Definition

litmus

noun

lit·​mus ˈlit-məs 

: a coloring matter from lichens that turns red in acid solutions and blue in alkaline solutions and is used as an acid-base indicator