knife-edge

Short definition from Wiktionary

Noun

1. 1
   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.
2. 2
   A precarious balance that could be upset by a very small force in either direction.
3. 3
   Used other than figuratively or idiomatically: the edge of a knife.
4. 4
   Alternative form of knife-edge.