PA = Powe Amplifier. It turns a voltage swing across a high impedance ( Pin = V^2/R) into a voltage swing across a lower impedance. It was a good term to use in the days of the thermionic valve where Pin was miniscule (very high input impedance) and Pout was perhaps 1000 times greater. With the advent of the junction transistor, with low input imedances, hence significent input power, the overall power gain could easily be as low as 3. MOSFETs with their high imput impedance, have got back to being more like a valve. PA is the term really for the output stage of a HiFi set or a transmitter rather then used for signal manipulation.
Frank