n.
Pronunciation: ' lit-m ə s
Function: noun
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 m ō s moss
Date: 14th century
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