The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

   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.
         [1913 Webster]

   Litmus paper (Chem.), unsized paper saturated with blue or
      red litmus, -- used in testing for acids or alkalies.
      [1913 Webster]

WordNet (r) 3.0 (2006):

litmus
    n 1: a coloring material (obtained from lichens) that turns red
         in acid solutions and blue in alkaline solutions; used as a
         very rough acid-base indicator [syn: litmus, litmus
         test]