Blank

_ or Blank[] is a pattern object that can stand for any expression.

_h or Blank[h] can stand for any expression with head h.

Examples

Match any expression:

MatchQ[5, _]
(* True *)

Match specific head:

MatchQ[5, _Integer]
(* True *)

MatchQ[5.0, _Integer]
(* False *)

Please visit the official Wolfram Language Reference for more details.