The primary and secondary cylinders are what translate the motion of the pedal into the hydraulic medium and then into physical motion of the calipers. Inside the caliper is the secondary cylinder that moves back and forth when pressed on by the hydraulic fluid.

In brakes it’s usually called primary and piston or master and piston though.

Even the 90 year old guy and the transmission shop didn’t say master and slave cylinder when they were putting new clutch plates in my truck a couple of years ago.