What is "HRC"?
The Acronym "HRC" actually stands for "Hardness on Rockwell Scale C."
The Rockwell family (Hugh M. Rockwell and Stanley Rockwell (1886–1940)) from the US co-invented the "Rockwell hardness tester," a differential-depth machine. Their company grew to build and patent numerous hardness testers. The machine used for steel used in knives is the Scale C tester - hence HRC.
The machine itself applies downward force onto any steel being tested with two diamond indents. From this test it measures the hardness of a steel by looking at the depth of indentation.