basically it's a high performance street cam. it comes from the old school flat head era.
there was "semi race" and "full race" grinds and what they did was grind the intake lobe as full race and the exhaust as semi race, thus it was only 3/4 race.
basically today it kinda encompasses a whole slew of performance cams that are somewhere in the realm of that 3/4 of the way in between hotter than stock and not full race. sometimes people will refer to them as "stage 3" as well.
legendary cam grinders Winfield and Harman-Collins were the ones that are credited with it.