If I want to control a motor by simply giving it a variable specific voltage or current, to control the RPM, not by flipping the power on and off as in pulse width modulation, what is that called?
Controlling a DC motor by simply controlling the voltage is called armature voltage control or adjustable voltage control. That is not really the opposite of pulse width modulation. It is a different way of doing the same thing. You might say it is an analog method rather than a digital method.
Controlling the current is a method of controlling torque. That can be used for speed control by adjusting the current in response to a speed error signal.
Controlling a DC motor using a controlled rectifier (thyristor) is sometimes called pulse amplitude control. More often, it is called SCR (silicon controlled rectifier) control.
For AC motors, voltage control is quite limited. Frequency control is preferred.