Keys

Keys[assoc] gives a list of the keys in an association.

Keys[{key1 -> val1, key2 -> val2, ...}] gives a list of keys in a list of rules.

Examples

Get keys from an association:

Keys[<|"a" -> 1, "b" -> 2, "c" -> 3|>]
(* {"a", "b", "c"} *)

Get keys from rules:

Keys[{x -> 1, y -> 2}]
(* {x, y} *)

Please visit the official Wolfram Language Reference for more details.