STAB stands for Same-Type Attack Bonus. When a Pokémon uses a move whose type matches one of its own types, the move deals 1.5× more damage.
For example, a Charizard (Fire / Flying) using Flamethrower gets STAB because Flamethrower is a Fire-type move and Charizard is a Fire-type Pokémon. The same Charizard using HurricaneHurricanemoveA Flying-type special move with 110 base power — normally 70% accurate, but 100% accurate in rain.Click to read more → also gets STAB because Hurricane is a Flying-type move.
A Pokémon using a move that doesn’t match either of its types gets no bonus. So a Charizard using Earthquake (a Ground-type move) deals normal damage.
Why it matters
STAB is one of the most important factors in damage calculation. When you’re choosing moves for a Pokémon, your same-type attacks are almost always the highest-damage option you have access to. Coverage moves (different-typed moves used to hit otherwise-resistant opponents) are typically only chosen when STAB moves can’t get the job done.
Special case: Adaptability
The ability AdaptabilityAdaptabilityabilityDoubles the STAB bonus from 1.5× to 2× — matching-type moves hit harder than on any other Pokémon.Click to read more → doubles the STAB bonus from ×1.5 to ×2. This is rare — only a handful of Pokémon get it.
In doubles
STAB interacts with spread movesSpread MovesmechanicMoves that hit both opposing Pokémon simultaneously, at 0.75× damage — the defining mechanic of doubles offense.Click to read more → in a way that amplifies their value enormously. A spread move already hits both opposing Pokémon at once. If that move also gets STAB, you’re dealing 1.5× damage to both targets simultaneously.
The classic example: a Fairy-type Pokémon using Dazzling Gleam (a Fairy-type spread move) against two Dragon-type opponents deals 1.5× STAB to both of them at the same time. One move, two targets, one and a half times the damage on each. This is a core reason why offensive typing matters so much in doubles team-building — good STAB spread moves apply pressure to the entire opposing side of the field at once.