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. However, there is a disadvantage to the infinite uses: a move learned through an HM cannot be forgotten without the aid of the Move Deleter.
A mutually compatible HM move be passed down through breeding with the male Pokémon passing down moves in the egg groups, much like mutually compatible TMs can be. Only four Pokémon learn an HM move through leveling up.
Although HM moves are neccessary to complete the games (except for Flash, though it is very helpful), many of them lack usefulness in actual Pokémon battles. Thus, players often capture extra Pokémon to keep in their parties solely to use the HM moves. However, Surf (HM03) is one of the stronger Water-type moves.
The original five HMs remained the same from Generation I up to Generation III, as in Generation IV, Flash was moved from its position as HM05 to TM70, leaving the space open for Fog Clear. What remains true, nonetheless, is that all HMs except for HM05 contain damage-dealing moves in every generation released so far.