- the edge of a knife
- any very sharp edge
- a metal wedge whose fine edge serves as the fulcrum for a scale beam, pendulum, etc.
- A sharp cutting edge.
- A sharp, narrow edge or border: “saying lines that teeter on the knife-edge between literature and lunacy” (Vincent Canby).
- A wedge of metal used as a low-friction fulcrum for a balancing beam or lever.
- A piece of steel sharpened to an acute edge or angle, and resting on a smooth surface, serving as the axis of motion of a pendulum, scale beam, or other piece required to oscillate with the least possible friction.
- (figuratively) A precarious balance that could be upset by a very small force in either direction.
- Used other than as an idiom: the edge of a knife.