invariant

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invariant

[in′ver·ē·ənt]
(mathematics)
An element x of a set E is said to be invariant with respect to a group G of mappings acting on E if g (x) = x for all g in G.
A subset F of a set E is said to be invariant with respect to a group G of mappings acting on E if g (x) is in F for all x in F and all g in G.
For an algebraic equation, an expression involving the coefficients that remains unchanged under a rotation or translation of the coordinate axes in the cartesian space whose coordinates are the unknown quantities.

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.