What defines where a person lives.

In the arena you are discussing nomenclature, or the exact name or label, you use makes all the difference.  You should look carefully at you lease.  It may we speak to who is to "occupy" the premises, rather than who "lives" there.  Generally the words "residence" or "domicile" involve a combination of physical presence and subjective mental intent.  These words refer to where one permanently abides.  Your girlfriend may have a strong claim her parents' home is her residence or domicile because she intends for it to be her permanent home, and if she is away, she intends to return.  One can be physically away from their residence without it ever losing its character as its residence, such  as when a soldier goes away to war, or a business person is posted in another city, state, or even overseas to work for their employer.  If they intend to return, their residence stays their residence. Who is occupying an apartment is a function of who is physically present.  Intent is not involved.  Your girlfriend could be occupying your apartment without ever losing her parents' home as her residence. The word "living" if that term is really what is being used is vague but probably does not involve intent.  I think for example a soldier could say, "My residence is Oklahoma, but aI am living in Afghanistan while I am stationed there." Check  exactly what your lease says about the number of people present in the apartment.  In this issue words do matter.