A spell to revive someone who is stunned.

Enervate is a spell that is used to restore a person to consciousness, especially if he/she has been Stunned. Later versions of the series have been changed to indicate that the incantation is Renervate.

Enervate seems to be a spell of limited usefulness; we only ever see it being used when resuscitating someone from a Stunning Spell. Possibly it could be used when restoring consciousness to someone who has been knocked out by e.g. a blow to the head, but we never see such a use.