Protect

Protect[s1, s2, ...] sets the attribute Protected for the symbols, preventing modification of their definitions.

Examples

Protect a symbol:

f[x_] := x^2;
Protect[f]
(* f is now protected *)

f[x_] := x^3
(* Set::write: Tag f is Protected *)

Check if protected:

MemberQ[Attributes[Plus], Protected]
(* True *)

Please visit the official Wolfram Language Reference for more details.