HM

From the Pokémon Wiki, a database for the Pokémon series that anyone can contribute to

A Hidden Machine, HM for short, is in almost every way identical to a TM, except for three differences: the moves cannot be overridden by simply learning another move, the moves can be used outside of battle after obtaining the relevant gym badge, and the HM can be used multiple times without breaking. Additionally, Pokémon on a third generation game cartridge cannot be transferred to a fourth generation game card if it knows a HM move, you must go to the Move Deleter in your game in order to transfer these Pokémon to Pokémon Diamond/Pokémon Pearl.