The ability holder's accuracy cannot be lowered, such as from moves like Smokescreen.
Generation III - Prevents loss of accuracy.
Generation IV - Prevents the Pokémon from losing accuracy.
Outside of Battle
From Pokémon Emerald onwards, if the ability holder is leading the party, wild Pokémon that are 5 levels or more below the user's Pokémon have a 50% chance of not being encountered. It shares this effect with Intimidate.
Pokémon with the Ability
As a Hidden Ability