knife-edge

Meanings

1. A small piece of metal, usually steel, sharpened to an acute edge or angle, and resting on a smooth surface, serving as the fulcrum of a pendulum, scalebeam, or other piece required to oscillate with minimal friction.
2. A precarious balance that could be upset by a very small force in either direction.
3. Used other than figuratively or idiomatically: the edge of a knife.