It's quite common in the US, but not in British English. (Note that the corresponding use of "boyfriend" does not exist.)

In America, women here often call their female friends "girlfriends". Example: "My girlfriend and I were at the mall the other day, and suddenly we ran into her girlfriend! It was amazing..." However, women do not call their male friends "boyfriends". Note: Men neither use "Girlfriend" or "Boyfriend" when talking about their friends.

'Hey Girlfriend' is kind of a phrase in itself, it's hard to explain but it's meant to sound 'sassy'. It's a common phrase so most people will know what you mean. Since you are speaking directly to that person, and they already know they are not your girlfriend, there is no confusion. I also hear people saying 'I went to the cinema with my girlfriends', and again, since you obviously don't have multiple girlfriends there is no confusion, so it's fine to say that. But if you said 'I went to the cinema with my girlfriend', that might be misunderstood because the person doesn't know if you mean a female friend or a lover. But it is an American term to call your female friends 'girlfriends', in England we don't say that at all but some people would know what you meant because we also hear it on American TV shows.

Yes, you may! It depends upon the context. The comment you are referring to from "The Big Bang Theory" is probably slang. If you can provide me a URL link to this episode on YouTube, I'll be glad to take a look at it and respond in more detail. (Note: Use of the term 'girlfriend' is vernacular English.) The word "girlfriend" (and "boyfriend") are broad concepts in spoken English today; much as the word "love" is used in modern culture. Summary: It depends upon the context. Meanings (in American culture): 1. "Girlfriend" usually means an exclusive dating relationship between lovers. (This refers to a "pre-engagement" type of dating relationship.) {Primary meaning.} 2. "Girlfriend" can also mean casual friendship. (This reference is used primarily between female roommates, classmates, office-mates, etc.) {Secondary meaning.} 3. Slang expressions using "girlfriend" are not uncommon. (Yet, slang expressions typically dilute the meaning of the word; or they are a parody of the word "girlfriend".) {Third meaning.} 4. "Girlfriend" may sometimes [rarely] be used between boys and girls who are strictly friends — in the Platonic sense. However, in this context: they would have been "best friends" from high school, or "friends of the family", or have become friends through marriage of a close friend, etc. (Note: The circumstances underlying the use of "girlfriend" in this context are unique, and exceptionally rare in American culture.) {Hybrid meaning.} Hope this is helpful to you! Sorry this is such a formal answer! :( Let me know if I can provide better clarity... BTW: I'm an American (by birth). Native English speaker. Living in the USA. I work as a corporate trainer in computer engineering & architecture. ~ Pardon the fragments! ;) lol

When using "girl friend", it is advisable to use it when it is clear what it means. If you would tell to a stranger that "My girl friend cooked for me last night", then that might be understood in a different way than you meant it. However, if you said "I'm meeting my girl friends tonight" then there would be less ambiguity.