Other forms: about-faces; about-faced; about-facing
An about-face is a turn in the opposite direction. If you're halfway to school when you realize you forgot your lunch, you may have enough time to make an about-face, run home, and grab it.
A literal about-face, when you pivot your body to face 180 degrees away from where you started, has a military origin. About-face is a drill command that's given to a group of marching soldiers, who must turn in a smooth, synchronized way to face the opposite direction. Figuratively, an about-face can be an abrupt change of belief, opinion, or practice — like your friend whose diet leans heavily on fast-food hamburgers until he does an about-face and becomes a vegetarian.