The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. The plant from which the dye is obtained. [Written also
      orchal and orchil.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

archil
    n 1: a purplish dye obtained from orchil lichens [syn: orchil,
         archil, cudbear]
    2: any of various lecanoras that yield the dye archil [syn:
       archil, orchil]