knife-edge

plural knife-edges · alternative knife edge · alternative knife's edge · alternative knife's-edge

1. engineeringA 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 28, James Howard Gore, “The Shape of the Earth from a Pendulum”, in Popular Science Monthly, volume 44

2. figurativelyA precarious balance that could be upset by a very small force in either direction.

   The game was on a knife-edge.

   3 more examples

   […]meanwhile, Biloxi and Vincennes are both in the process of going down and being abandoned, whilst Miami is right on the knife-edge of being recoverable, with three destroyers clustering around offering pumping and additional damage-control crews to try and keep the light cruiser afloat.

   2021 May 5, Drachinifel, 43:40 from the start, in Battle of Samar - What if TF34 was there?, archived from the original on 19 Aug 2022

   Boris Johnson's future on a knife edge after No 10 party apology [title]

   2022 January 12, “Boris Johnson's future on a knife edge after No 10 party apology”, in The Guardian

   Polling stations have opened in a knife-edge election in Finland, with polls suggesting the three leading parties are running neck-and-neck and prime minister Sanna Marin may face an uphill battle to stay in power.

   2023 April 2, Jon Henley, “Finland begins voting in knife-edge election”, in The Guardian, →ISSN

3. Used other than figuratively or idiomatically: the edge of a knife.