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.

Roccellic \Roc*cel"lic\, a. [F. roccellique, fr. roccelle
   archil, It. & NL. roccella, fr. It. rocca a rock, because
   archil grows on rock.] (Chem.)
   Pertaining to, or designating, a dibasic acid of the oxalic
   series found in archil ({Roccella tinctoria}, etc.), and
   other lichens, and extracted as a white crystalline substance
   {C17H32O4}.

Archil \Ar"chil\ (?; 277), n. [OF. orchel, orcheil, It. orcella,
   oricello, or OSp. orchillo. Cf. {Orchil}.]
   1. A violet dye obtained from several species of lichen
      ({Roccella tinctoria}, etc.), which grow on maritime rocks
      in the Canary and Cape Verd Islands, etc. --Tomlinson.

   2. The plant from which the dye is obtained. [Written also
      {orchal} and {orchil}.]

Source : WordNet®

Roccella tinctoria
     n : a source of the dye archil and of litmus [syn: {roccella}]