It really does not matter whether you call it AC ground or DC ground or something else, because, ultimately as per Kirchoff's law all currents that left the + terminal of the supply will have to go back to its negative terminal. I would like to classify ground as Power Ground, Signal Ground, Analog Ground, Digital ground etc.
Let me explain what I meant by each of the above grounds and its significance as I see them:
Power Ground: All return points of high Power supply and its bypass capacitors and portions of circuit delivering high power should be preferably separated from low level analog and logic circuits to avoid potential drops in the ground line of low level circuits, to avoid noise in the system and difficult to trace spurious current feed back and instability, easpecially in high gain op-amp circuits. Now think of the returns of the bypass capacitors for emitter resistors and op-amp suppies, since these carry only small currents, they should be connected to the signal ground only. Further, if these capacitors are returned to the noisy power ground, it will be equivalent to injecting noise direcly to the emitter of transistor and in case of an op-amp power supply, the supply lead will show up the noise of the power ground with respect to the signal ground, the opposite of what we wanted.
This ground is where you should return all signal -ves, signal bypass negatives and Supply bypass capacitors of low power, high gain devices.
For instance in an audio amplifier, all grounds of preamplifier circuits including microphone return will go to signal ground, all grounds of power amplifier circuits including Speaker return will go to Power ground.
Analog Ground : Same as signal ground, but consists of all returns of low power analog circuits.
Digital ground: All returns of low power logic circuits.
Depending on the complexity of the board, you may have all the four types of grounds on a single board. But remember, if the signals on the board are not ISOLATED, ULTIMATELY ALL GROUNDS SHOULD be SHORTED at One point, since as per KIRCHOFF's law a current leaving one portion of the circuit SHOULD always return to it, however small the current may be.
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