Stamina is a stat in Adventure High.
The more Stamina you have left, the sooner your turn comes around. Using a Spell depletes some Stamina, with higher leveled spells depleting more compared to lower leveled ones.
There's currently two ways to recover Stamina.
The first is to exit the Dungeon and head to the Protagonist's room.
The second is to use the Rest command during battle. It recovers a small amount of HP and a small amount of Stamina, but it takes quite some time for that character to act again.