The Collaborative International Dictionary of English v.0.48:

Premature \Pre`ma*ture"\, a. [L. praematurus; prae before +
   maturus ripe. See Mature.]
   1. Mature or ripe before the proper time; as, the premature
      fruits of a hotbed.
      [1913 Webster]

   2. Happening, arriving, existing, or performed before the
      proper or usual time; adopted too soon; too early;
      untimely; as, a premature fall of snow; a premature birth;
      a premature opinion; premature decay.
      [1913 Webster]

   3. Arriving or received without due authentication or
      evidence; as, a premature report.
      [1913 Webster] -- Pre`ma*ture"ly, adv. --
      Pre`ma*ture"ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

premature
    adj 1: born after a gestation period of less than the normal
           time; "a premature infant" [ant: full-term]
    2: too soon or too hasty; "our condemnation of him was a bit
       previous"; "a premature judgment" [syn: previous(p),
       premature]
    3: uncommonly early or before the expected time; "illness led to
       his premature death"; "alcohol brought him to an untimely
       end" [syn: premature, untimely]

Moby Thesaurus II by Grady Ward, 1.0:

69 Moby Thesaurus words for "premature":
   a bit previous, advanced, beforehand, early, far ahead, forward,
   green, half-baked, half-cocked, hasty, ill-considered,
   ill-seasoned, ill-timed, immature, improper, impulsive,
   inappropriate, inauspicious, inconvenient, inexpedient,
   infelicitous, inopportune, intempestive, intrusive, irrelevant,
   late, mal a propos, malapropos, mistimed, not firm, off base,
   out of line, out of phase, out of time, overhasty, oversoon,
   precipitate, precocious, previous, rushed, soon, too early,
   too late, too soon, unbefitting, uncrystallized, underdeveloped,
   undeveloped, unfavorable, unfit, unfitting, unfledged, unfortunate,
   unhandy, unhappy, unjelled, unlucky, unmatured, unmeditated,
   unpremeditated, unprepared, unpropitious, unready, unripe,
   unseasonable, unsuitable, untimely, untoward, wrong