Stamina is an attribute that allows a certain character to remain active, exert itself to prolonged periods of time, and recover from ordeals.
Take note that we have no set categories for describing this attribute, so you should preferably rather describe the stated or displayed extent for a certain character.
For example: "The official Marvel handbooks have described Thor's stamina as virtually inexhaustible. He once fought an army of frost giants for 9 months without tiring."
Infinite stamina should only apply to characters that can use their abilities any number of times without exhaustion.