<mathematics> An equivalence class is a subset whose elements
are related to each other by an equivalence relation. The
equivalence classes of a set under some relation form a
partition of that set (i.e. any two are either equal or
disjoint and every element of the set is in some class).

<testing> A software testing technique that involves
identifying a small set of representative input values that
invoke as many different input conditions as possible.

For example, for binary search the following partitions
exist: inputs that do or do not conform to pre-conditions,
Inputs where the key element is or is not a member of the
array. One can combine these into finer partitions. One can
also pick specific conditions of the array, e.g. a single
value, even or odd number of elements. One should look at
boundary conditions, e.g. inputs where the key element is
the first or last element in the array.

<mathematics> A relation R on a set including elements a, b,
c, which is reflexive (a R a), symmetric (a R b => b R a) and
transitive (a R b R c => a R c). An equivalence relation
defines an equivalence class.