Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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}]

Source : Free On-Line Dictionary of Computing

invariant
     
         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)