The Collaborative International Dictionary of English v.0.48:

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]

WordNet (r) 3.0 (2006):

invariant
    adj 1: unaffected by a designated operation or transformation
    2: unvarying in nature; "maintained a constant temperature";
       "principles of unvarying validity" [syn: changeless,
       constant, invariant, unvarying]
    n 1: a feature (quantity or property or function) that remains
         unchanged when a particular transformation is applied to it

The Free On-line Dictionary of Computing (30 December 2018):

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)