The Collaborative International Dictionary of English v.0.48:

Lithium \Lith"i*um\ (l[i^]th"[i^]*[u^]m), n. [NL., from Gr.
   li`qeios of stone, fr. li`qos stone.] (Chem.)
   A metallic element of the alkaline group, occurring in
   several minerals, as petalite, spodumene, lepidolite,
   triphylite, etc., and otherwise widely disseminated, though
   in small quantities.
   [1913 Webster]

   Note: When isolated it is a soft, silver white metal,
         tarnishing and oxidizing very rapidly in the air. It is
         the lightest solid element known, specific gravity
         being 0.59. Symbol Li. Atomic weight 7.0 So called from
         having been discovered in a mineral.
         [1913 Webster]

WordNet (r) 3.0 (2006):

lithium
    n 1: a soft silver-white univalent element of the alkali metal
         group; the lightest metal known; occurs in several minerals
         [syn: lithium, Li, atomic number 3]