A quantity which remains unchanged under certain classes of transformations. Invariants are extremely useful for classifying mathematical objects because they usually reflect intrinsic properties of the object of study.

Hunt, B. "Invariants." Appendix B.1 in The Geometry of Some Special Arithmetic Quotients. New York: Springer-Verlag, pp. 282-290, 1996.

Olver, P. J. Classical Invariant Theory. Cambridge, England: Cambridge University Press, 1999.

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Weisstein, Eric W. "Invariant." From MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/Invariant.html