HM05 (Japanese: ひでんマシン05Secret Machine 05) is an HM introduced in Generation I. It teaches a compatible Pokémon a move that can also be used in the field; but using an HM in the field requires that the player have a certain Badge (except in Generation V). The move it teaches depends on the game. For compatibility, refer to the move's page. HMs can used to teach moves without limit.
HMs in the Mystery Dungeon series are named only by the move they teach. For example, Cut is the item that teaches the move Cut. Most HMs, however, correspond to an HM in the core series' contemporaneous games.
HMs in Pokémon Mystery Dungeon games prior to Pokémon Super Mystery Dungeon can be used any number of times. In Pokémon Super Mystery Dungeon, "HMs" are treated like TMs and only usable once. However, if an "HM" is dropped on the ground, it has a different color than a TM.
HM05 appears as a Water-type HM in Diamond, Pearl, and Platinum.
In Diamond, Pearl, and Platinum, HM05 has the item sprite of a Water-type HM, even though Defog is a Flying-type move. It is possible that a Water-type HM was considered for the position before Defog was finalized. Whirlpool, a Water-type move itself, takes the spot in HeartGold and SoulSilver, though this is likely unrelated, as HeartGold and SoulSilver came out more than three years after Diamond and Pearl were released.
HM05 is the only HM that has changed moves within a generation, from Defog in Diamond, Pearl, and Platinum to Whirlpool in HeartGold and SoulSilver.
HM05 has contained the most different moves of any HM, teaching four different moves throughout the series.
Interestingly, two of the moves it has contained are the two HM moves which first appeared as HMs in Generation II.
HM05 is the first and so far the only HM that has been handed out as a prize by a Gym Leader when defeated.