Same-type attack bonus

Same-type attack bonus, commonly abbreviated as STAB, is a 50% boost to an attack's power when the attack's type is the same as one of the types of the Pokémon using the attack. For instance, if a Blastoise were to use Surf, Surf's power would be increased by 50% since both Blastoise and Surf are Water-type. If Blastoise were to use Ice Beam however, it would receive no such bonus since Blastoise is not Ice-type. A Lapras, on the other hand, would receive the same-type attack bonus when using either Surf or Ice Beam since Lapras is both Ice-type and Water-type.

Contrary to some of Nintendo's official guides, Normal-type Pokémon do in fact receive the same-type attack bonus when using Normal-type attacks, and same-type attack bonus is not reduced for a Pokémon with two types.

Hidden Power does receive same-type attack bonus. A Fire-type Pokémon will receive same-type attack bonus when using a Fire-type Hidden Power. Although Hidden Power is Normal-type in the game's data, a Normal-type Hidden Power is not really obtainable. Because of this, a Normal-type Pokémon will never receive same-type attack bonus when using Hidden Power.

Same-type attack bonus does not apply to moves that do a set amount of damage and do not use the normal damage formula, such as Dragon Rage and Seismic Toss.

Additonally, if a Pokemon has 2 types (such as Crobat), the bonus from those types is lowered. For example, a Crobat using Poison Fang, would only recieve a 25% bonus because it needs to share the bonus with its dual type.