Source : Webster's Revised Unabridged Dictionary (1913)

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 : WordNet®

litmus paper
     n : unsized paper treated with litmus for use as an acid-base
         indicator