Slot

# (or #1) represents the first argument in a pure function. #n represents the nth argument. #name accesses keys in associations.

Examples

Use slots in pure functions:

(* First argument *)
(#^2 &)[5]
(* 25 *)

(* Multiple arguments *)
(#1 + #2 &)[3, 4]
(* 7 *)

(* Named slots with associations *)
(#name &)[<|"name" -> "Alice", "age" -> 30|>]
(* "Alice" *)

Please visit the official Wolfram Language Reference for more details.