Hidden Machines

Hidden Machines, often shortened to HM, are used to permanently teach a Pokémon a move that effects the outside world. Though only five are regularly obtainable, many more have been speculated to lead to the PokéGods.

Origin
HMs effect the areas where the player can go, meaning extra HMs could allow movement past other obstacles. This cause rumors to circulate about more HMs that could be used to go to a PokéGod City. Some of these are strikingly similar to real HMs that have been introduced since, like Dive and Rock Smash.