Litmus \Lit"mus\, n. [D. lakmoes; lak lacker + moes a thick preparation of fruit, pap, prob. akin to E. meat: cf. G. lackmus. See Lac a resinous substance.]

(Chem.) A dyestuff extracted from certain lichens (Roccella tinctoria, Lecanora tartarea, etc.), as a blue amorphous mass which consists of a compound of the alkaline carbonates with certain coloring matters related to orcin and orcein.

Note: Litmus is used as a dye, and being turned red by acids and restored to its blue color by alkalies, is a common indicator or test for acidity and alkalinity.

Litmus paper (Chem.), unsized paper saturated with blue or red litmus, -- used in testing for acids or alkalies.

Source: Webster's Revised Unabridged Dictionary (1913)