Definitions for knife edge
Random House Webster's College Dictionary
the cutting edge of a knife.
anything with a sharp edge.
a wedge on the fine edge of which a scale beam, pendulum, or the like, balances or oscillates.
Origin of knife edge:
a narrow boundary
"he lived on a knife-edge between genius and insanity"
knife edge, cutting edge(noun)
the sharp cutting side of the blade of a knife