HM
A Hidden Machine or HM is a machine used by Pokémon trainers to teach a Pokémon a new technique, or move that it would not otherwise learn. All HM moves are usable out of battle (though the reverse is not true), and all are essential for completing the game. Whereas TMs are good for only one use, HMs can be used over and over again on many different Pokémon.
A HM move cannot be forgotten without the aid of the Move Deleter. A HM move be passed down through breeding with the male Pokémon passing down moves in the egg groups.