Pattern

sym:obj or Pattern[sym, obj] represents the pattern object obj, assigned the name sym.

Examples

Named pattern:

f[x_Integer] := x^2
f[5]
(* 25 *)

Pattern with condition:

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

Please visit the official Wolfram Language Reference for more details.