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):

prematureness
    n 1: the state of being premature [syn: prematureness,
         prematurity]