Block

Block[{x, y, ...}, expr] specifies that expr is to be evaluated with local values for the symbols x, y, ....

Block[{x = x0, ...}, expr] defines initial local values for x, ....

Example

Temporarily change a global variable:

x = 10;
Block[{x = 5}, x^2]
(* 25 *)
x
(* 10 - unchanged outside Block *)

Please visit the official Wolfram Language Reference for more details.