The Collaborative International Dictionary of English v.0.48:

Obsolete \Ob"so*lete\, a. [L. obsoletus, p. p. of obsolescere.
   See Obsolescent.]
   1. No longer in use; gone into disuse; disused; neglected;
      as, an obsolete word; an obsolete statute; -- applied
      chiefly to words, writings, or observances.
      [1913 Webster]

   2. (Biol.) Not very distinct; obscure; rudimental;
      imperfectly developed; abortive.
      [1913 Webster]

   Syn: Ancient; antiquated; old-fashioned; antique; old;
        disused; neglected. See Ancient.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Obsolete \Ob"so*lete\, v. i.
   To become obsolete; to go out of use. [R.] --Fitzed. Hall.
   [1913 Webster]

WordNet (r) 3.0 (2006):

obsolete
    adj 1: no longer in use; "obsolete words" [syn: disused,
           obsolete]

The Devil's Dictionary (1881-1906):

OBSOLETE, adj.  No longer used by the timid.  Said chiefly of words.
A word which some lexicographer has marked obsolete is ever thereafter
an object of dread and loathing to the fool writer, but if it is a
good word and has no exact modern equivalent equally good, it is good
enough for the good writer.  Indeed, a writer's attitude toward
"obsolete" words is as true a measure of his literary ability as
anything except the character of his work.  A dictionary of obsolete
and obsolescent words would not only be singularly rich in strong and
sweet parts of speech; it would add large possessions to the
vocabulary of every competent writer who might not happen to be a
competent reader.