Element

Element[x, dom] or x ∈ dom asserts that x is an element of the domain dom.

Element[x, reg] asserts that x is an element of the region reg.

Examples

Assert integer domain:

Simplify[Sin[n Pi], Element[n, Integers]]
(* 0 *)

Real assumption:

Integrate[1/(1 + x^2), {x, -Infinity, Infinity}, Assumptions -> Element[x, Reals]]
(* Pi *)

Please visit the official Wolfram Language Reference for more details.