Complexes represents the domain of complex numbers, as in x Complexes.
x Complexes evaluates immediately only if x is a numeric quantity.
Simplify[expr Complexes] can be used to try to determine whether an expression corresponds to a complex number.
The domain of real numbers is taken to be a subset of the domain of complex numbers.
Complexes is output in TraditionalForm as .
See The Mathematica Book: Section 3.3.10.
See also: Element, Simplify, NumberQ, NumericQ, Complex, Reals.
Related package: Algebra`Quaternions`.