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