Knife edge
Knife edge turns up in 2 fields: vacuum, mechanical. Here is what it means to the people in each one.
Knife edge in vacuum
Sharp circular ridge machined into a CF flange face that cuts into the copper gasket to make the seal.
The angled sealing ridge inside the face of a CF flange, recessed below the flange surface so it is partly protected. On assembly it plastically deforms the copper gasket and forms a metal-to-metal seal. A nick or radial scratch across the edge, from a dropped flange or a screwdriver, will leak, so flanges are stored with caps on and never set face-down; a shallow ding can be recut on a lathe, while a deep one means a new part.
"Don't set that flange face-down on the bench, ding the knife edge and we're scrapping a whole nipple."
Knife-edge in mechanical
A sharp, hardened contact point used as a low-friction fulcrum.
A wedge-shaped piece of hardened steel or jewel that rests its sharp edge against a flat or V-shaped bearing surface. It acts as a pivot point that supports a load while allowing oscillation with nearly zero friction. These pivots are heavily utilized in precision weighing scales, pendulums, and sensitive mechanical triggers where rolling or sliding friction would ruin accuracy.
"The scale is out of calibration because dust built up on the knife-edge pivot."