\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.