knife edge

noun

1. the cutting edge of a knife.
2. anything very sharp.
3. a wedge on the fine edge of which a scale beam, pendulum, or the like, balances or oscillates.

noun

1. the sharp cutting edge of a knife
2. any sharp edge
3. a sharp-edged wedge of hard material on which the beam of a balance pivots or about which a pendulum is suspended
4. a critical point in the development of a situation, process of making a decision, etc