Don't know about threshold speeds???
Vmca - Minimum Control Speed in the Air
Vmcg - Minimum Control Speed on the Ground.
Hence Vmcg is the minimum speed from which the pilot will be able to maintain directional control to continue the take off, all other performance factors being satisfied. Quite obviously below Vmcg you must stop or go off the runway.
There are, of course, various configurations defined for the testing. JT and Mutt can, I'm sure, expand on this.