PatternTest

p?test is a pattern object that stands for any expression that matches p, and on which the application of test gives True.

Examples

Match only integers:

f[x_?IntegerQ] := x^2;
f[5]
(* 25 *)

Match only positive:

g[x_?Positive] := Sqrt[x];
g[4]
(* 2 *)

Please visit the official Wolfram Language Reference for more details.