To make something twice as much or as many. The company plans to double its production output by next year.
To fold or bend something so that one part lies on top of another part of it. She doubled the piece of paper before putting it in her pocket.
To serve in a second capacity or role. He doubles as a teacher and a coach at the school.
To replace or stand in for someone temporarily. The actor had to double for the lead during the action scenes.