From WordNet (r) 2.0 (August 2003) [wn]:
invariant
adj 1: unaffected by a designated operation or transformation
2: persistent in occurrence and unvarying in nature;
"maintained a constant temperature"; "a constant beat";
"principles of unvarying validity"; "a steady breeze"
[syn: changeless, constant, steady, unvarying]
From The Collaborative International Dictionary of English v.0.48 [gcide]:
Invariant \In*va"ri*ant\, n. (Math.)
An invariable quantity; specifically, a function of the
coefficients of one or more forms, which remains unaltered,
when these undergo suitable linear transformations. --J. J.
Sylvester.
[1913 Webster]
From The Free On-line Dictionary of Computing (19 Sep 2003) [foldoc]:
invariant
<programming> A rule, such as the ordering of an ordered
list or heap, that applies throughout the life of a data
structure or procedure. Each change to the data structure
must maintain the correctness of the invariant.
(1996-03-12)