Note that in portable devices the term "ground" is used in place of "common reference point" which may cause confusion since it may be inferred that a stake needs driving into the earth's crust - which is not necessary.
And that my friend is the root of the problem and misunderstanding that many newcomers have when starting in electronics, and it's all our faults. We tend to overuse and misuse the term 'ground' when we really should call it circuit common or just common. The use of a circuit common is mostly a graphical aid to keep from having to draw all the circuit return paths to the voltage source in schematic drawings.
A true ground would be a connection that ends up at a ground rod somewhere which of course a battery powered circuit does not have or require (unless you are dealing with RF antenna circuits where you need a true ground for antenna efficiency). In fact any circuit that is powered from a transformer coupled power source (like a 'wall wart') does not require a true ground to operate, but might require it for electrical code requirements.
So most of us are guilty of 'overloading' the usage of the word 'ground', but it's so common (pun, yes?) in usage that nothing is bound to change or correct that usage.