(esp. in Australian English) A very close female friend often of another female. Does not necessarily imply any monogamous or sexual connection rather a strong social bond. Typically in this context used by one female to describe another and indicate a close friendship. Indistinguishable from the form "girl friend" in this use and can be written either way.
"I'm out shopping with my girlfriend"
"My girlfriend and I went to that concert"
To avoid ambiguity "My girlfriend and both our partners went to the pub on Saturday"