[lit.mus] n [ME litmose, of Scand origin; akin to ON litmosi herbs used in dyeing, fr. litr color (akin to OE wlite brightness) + mosi moss; akin to OE mos moss] (14c) 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