Defiant is a Pokémon ability introduced in the fifth generation. It increases the Pokémon's Attack when a stat is lowered.
Defiant raises the Attack of the Pokémon by 2 stages whenever a stat is lowered. It will not activate if a stat is lowered by itself (like when using Close Combat) or if lowered by an ally.
If this Pokémon is holding a White Herb, the ability will activate before using the White Herb. If it was attack that was lowered, Defiant will increase before activating the item, so it does not get used.