Rivalry is a Pokémon ability introduced in the fourth generation. It changes power of moves based on the gender of the Pokémon.
Pokémon with Rivalry increase the power of attacks by 25% if the opposing Pokémon is of the same gender. However, the power of attacks decrease by 25% if the opposing Pokémon has the opposite gender. Moves against genderless Pokémon are the same attack power. Prior to the fifth generation, Rivalry powered up confusion damage.