LITMUS

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 mōs moss Date: 14th century a coloring matter from lichens that turns red in acid solutions and blue in alkaline solutions and is used as an acid-base indicator, the critical factor in a ~ test