Pure Power doubles the Attack stat of the Pokémon with this Ability.
Because they use Attack modifiers (except stat stages), it also doubles the user's effective Attack stat when using Foul Play or Body Press.
Generation IV
Pure Power also affects self-inflicted confusion damage.
Generation V onward
Pure Power no longer affects confusion damage.
Pokémon Legends: Z-A
Because Abilities are not present in Pokémon Legends: Z-A, Pokémon that have Pure Power as their standard Ability have, instead, their base Attack stat increased by 40% exclusively in this game:
Meditite has its Attack stat increased from 40 to 56.
Medicham has its Attack stat increased from 60 to 84.
Mega Medicham has its Attack stat increased from 100 to 140.
Pure Power has a one-in-three chance of boosting a physical move (including regular attacks and items like Iron Thorn) by 50% when such an attack is used. The chance of activating appears to be lower in Pokémon Super Mystery Dungeon.