The Collaborative International Dictionary of English v.0.48:

Codeine \Co"deine\ (k[=o]"d[=e]n or k[-o]*d[=e]"[i^]n), n. [Gr.
   kw`deia poppy head: cf. F. cod['e]ine.] (Chem.)
   One of the opium alkaloids; a white crystalline substance,
   C18H21NO3, similar to and regarded as a derivative of
   morphine, but much feebler in its action; -- called also
   codeia.
   [1913 Webster]

WordNet (r) 3.0 (2006):

codeine
    n 1: derivative of opium; used as an antitussive (to relieve
         coughing) and an analgesic (to relieve pain)