I have a classmate and sometimes she says something like "I told my girlfriend that she has to blah blah".
Does this wording imply a lesbian relationship, or is she referring to a her best friend (a girl) only?
It's true that girls often call their close female friend(s) girlfriend(s), at least in the US. And although I am inclined to believe that the friend is likely a platonic female friend, it is still ambiguous.
1. A female companion or friend with whom one has a sexual or romantic relationship.
2. A female friend.
If you really care to know, you should ask her for clarification.
It depends where in the world you are.
Everywhere, as far as I know, one meaning of "girlfriend" is "a woman you're having a romantic and/or sexual relationship with but aren't married to." In the US, it's very common for women to describe close female friends as "girlfriends", even when there is no romantic or sexual involvement. In the UK, the friend-who-is-a-woman meaning is much less common than in the US and would be seen as something of an Americanism.
Having said that, the proportion of women in the UK who use "girlfriend" to mean any close female friend could easily be higher than the proportion of lesbian and bisexual women. If that's the case then a woman saying "my girlfriend" in the UK would still be more likely to mean "my close female friend" than "my romantic/sexual partner".
The other answers have done a great job, and I fully agree. If a girl refers to another girl as a "girlfriend" it could mean either way but it's very common to refer to just a female friend.
I wanted to add some additional information to say that this (for whatever reason) is very specific to one girl referring to another. If a girl said "boyfriend" most people would would assume that means a romantic relationship, and it would sound weird otherwise. I have heard "guy-friend" to refer to platonic male friends, but I'm not sure how common this is. (It might be regional)
Similarly, if I (a male) were to refer to someone as a "(girl|boy) friend" most people would infer that I mean a romantic partner. I probably would too.
I would never refer to a platonic friend as a "(girl|boy) friend", and it would sound very strange to hear someone doing so.
Unlike boys, girls very often call their close female friends girlfriends.
Even as a native speaker (a gay one, at that) this can be ambiguous. In my experience, using "girlfriend" to refer to friends is something that older women do and probably not the under-thirty set that is more used to gay culture. However, I've always lived in liberal, very gay-friendly environments so I wouldn't be shocked if I found that young women in less LGBT-friendly American regions used "girlfriend" as a synonym for "friend". If understanding were crucial to your conversation, I think asking "Is she just a friend or are you two dating?" would be perfectly acceptable.
Often, if the ladies in question are in a romantic relationship then they will refer to each other as their partner (this is true for both genders).
As mentioned, that doesn't also preclude them from referring to each other as girlfriend as well.
Thank you for your interest in this question.
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