NIntegrate

NIntegrate[f, {x, xmin, xmax}] gives a numerical approximation to the definite integral.

Examples

Numerical integration:

NIntegrate[Sin[x]^2, {x, 0, Pi}]
(* 1.5708 *)

Double integral:

NIntegrate[x y, {x, 0, 1}, {y, 0, 1}]
(* 0.25 *)

Please visit the official Wolfram Language Reference for more details.