How is Ground in a Power Supply made?
Back in the stony ages when relays were first used for telegraphy, it was found that instead of making a circuit by running two copper lines from station to station, in some locations a long rod could be driven into the earth at each station and the earth between the rods used as one of the conductors, eliminating the cost of one copper wire.

Then, because Benjamin Franklin told everybody that electricity moved out from the arbitrarily named "positive" terminal of a battery and returned to the negative terminal, the batteries used to run the relays at telegraph stations were connected with the battery's switched positive terminal connected to the long copper lines stretched  between stations, and the return connected to the ground terminal.

The term stuck, and as time went by and circuits got more complicated, all of the components returning current directly to the battery's negative terminal were said to be connected to "ground", or "grounded".

Thus, "ground" became a reference against which voltages in the circuit were measured and, today, has come to mean the common point which all galvanically interconnected equipment shares, whether or not a direct connection to the Earth is made.