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

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

One of the best was the part about enjoying the season and not making playoff success your litmus for hockey fandom happiness. — Murat Ates, The Athletic, 25 Nov. 2024

Alpert maintains that his litmus for music has remained the same throughout the decades. — Gary Graff, Billboard, 26 Sep. 2024

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