Ground in AC circuit
Ground as a concept comes from referencing earth as a baseline voltage potential, that is, the ground. If you think about how mains wiring works, the AC signals vary with respect to each other, but they also refer to the earth as well - by design. That’s why there’s a grounding rod at the power feed to your house, that is tied to the neutral return.

To summarize, a single-phase AC feed will have four connections at the panel:


Line 1
Line 2 (inverse of line 1)
neutral (carries any imbalance between line 1 and 2)
earth ground (bonded together with neutral at the panel)


That said, there’s no specific reason for a DC or AC circuit to have a defined, fixed voltage with respect to earth - in fact, many systems don’t, they have a ‘floating ground’, which is kind of an oxymoron, but it means that the reference is only local to the circuit.

By convention, a local ‘ground’ is usually chosen from the voltage node that serves as the return path for all the voltages in the system.

And that reference can be literally any voltage node. For example, modern cars tie the (-) battery terminal to the car body as ground. But it wasn’t always this way - early cars and many British vehicles until the 1980s or so tied the battery (+) to the car body and called that ‘ground’.