"Girlfriend" is an overloaded function: It can mean anything from "a girl who is a friend of mine" to "my domestic partner." I have a woman in my life to whom I frequently refer as my girlfriend, and circumstances have given me reason to reflect upon what I mean when I say that word.
Perhaps the most important distinction, in my case, is that my referring to my girlfriend as my girlfriend is not referential to her being my possession, or to restrictions upon her behavior, or, for that matter, to any contract or covenant between us whatsoever, or anything like that.
Rather, what I mean by "girlfriend" is something like, "The woman whom I love, for who she is, to such an extent that I am romantically unavailable to anyone else. I am in love. I am madly in love, and have been for so long that I am confident in its robustness. I love everything about her: Her smile, her charm, her laugh, her jokes - often subtly edgy, sometimes not even subtly - the way that she kisses me, the way that her hand folds into mine, the way that we each know what the other is feeling or about to say before the other even notices the feeling or says it, her weird-ass memes, her gentle nature, her incredible beauty, her preternatural wisdom, her patience and kindness and pleasantly startling lust to squeeze every last drop of adventure and joy and discovery out of our time on this Earth beyond most mortals' wildest dreams, and . . . You get the idea."
In short, I mean that I love her completely - mind, body, and soul - in a way that I don't think most humans ever get to experience in a lifetime.
I feel bad for them.