knife edge

Noun

knife-edge (plural knife-edges)

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.
   • Again, as the pendulum was provided with sharp knife-edges on opposite sides near its upper end, shaped like a V, on which it swung, the greater the weight of the pendulum the more wear there would be on these knife-edges.1894 February, James Howard Gore, “The Shape of the Earth from a Pendulum”, in Popular Science Monthly, volume 44
2. (figuratively) A precarious balance that could be upset by a very small force in either direction.
   • The game was on a knife-edge.
3. Used other than figuratively or idiomatically: the edge of a knife.