Regenerator is a Pokémon ability introduced in the fifth generation. It recovers the health of the Pokémon when it is switched out.
When a Pokémon with Regenerator is switched out, 1/3 of their health is recovered. When a Pokémon gains the ability through Trace and similar, it will regain the health upon switching out if it isn't suppressed beforehand. Heal Block does not prevent healing from this ability.