ClearAll

ClearAll[s1, s2, ...] clears all values, definitions, attributes, defaults, options, and messages for the symbols.

Examples

Clear everything about a symbol:

f[x_] := x^2;
Attributes[f] = {Listable};
ClearAll[f]
(* f is now completely undefined *)

Clear by pattern:

ClearAll["Global`*"]
(* Clears all user-defined symbols *)

Please visit the official Wolfram Language Reference for more details.