Refine

Refine[expr, assum] gives the form of expr that would be obtained if symbols in it were replaced by explicit numerical expressions satisfying the assumptions.

Refine[expr] uses default assumptions specified by any enclosing Assuming constructs.

Examples

Simplify with assumptions:

Refine[Sqrt[x^2], x > 0]
(* x *)

Without assumptions:

Refine[Abs[x], x < 0]
(* -x *)

Please visit the official Wolfram Language Reference for more details.