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A signature move (Japanese: 専用技 exclusive move) is a move that can normally only be learned by a single Pokémon, evolutionary family, or Legendary group within a given generation.
Starting in Pokémon Sword and Shield, because most Pokémon not in the regional Pokédex are unobtainable, some Pokémon apparently gain or lose their signature moves based on availability, even if that move would otherwise be non-exclusive (for example, Accelgor in Pokémon Sword and Shield is the only Pokémon that can learn Water Shuriken since Greninja is not present in those games).
Most signature moves are learned by leveling up. Despite being signature moves of a particular species, other Pokémon can still sometimes learn the move through other means. Some signature moves can also be inherited via breeding or are known by certain gift or event Pokémon. Additionally, Smeargle can learn most signature moves via its own signature move, Sketch.