The Collaborative International Dictionary of English v.0.48:

Precondition \Pre`con*di"tion\, n.
   A previous or antecedent condition; a preliminary condition.
   [1913 Webster]

WordNet (r) 3.0 (2006):

precondition
    n 1: an assumption on which rests the validity or effect of
         something else [syn: condition, precondition,
         stipulation]
    2: an assumption that is taken for granted [syn: given,
       presumption, precondition]
    3: a condition that is a prerequisite
    v 1: put into the required condition beforehand