From 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]
From 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