hi,
In my experience this can be little confusing.
Some engineers for example, refer to battery polarity as say, +V for one of the terminals and Gnd, -V, Common or 0v for the other.
I can give you an example on how to explain it:
Look at the battery on a car, you will see that one terminal of the battery
is connected the car chassis/metalwork. This is considered to be the
Ground or Common. The terminal thats connected to the car switch/fuse
box could be either the Neg or Pos terminal, it depends upon the car type.
Simply in this context, the ground is the terminal to which the other battery voltages are referenced to.
Hope this helps.
Regards
EricG