From The Collaborative International Dictionary of English v.0.48 [gcide]:
Lithium \Lith"i*um\ (l[i^]th"[i^]*[u^]m), noun [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.
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.
From WordNet (r) 3.0 (2006) [wn]:
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