The difference lies in their usage:
* "Girlfriend" is the more common and modern way to refer to a romantic partner, specifically a female one. It implies a close and often intimate relationship.
* "Girl friend" is an older and less common phrase. It can refer to any female friend, regardless of the nature of the relationship.
Here's an example:
* "My girlfriend is coming to visit." (implies a romantic relationship)
* "My girl friend from college is in town." (implies a friendship, not necessarily romantic)
While both phrases can technically be used to describe a female friend, "girlfriend" is the preferred choice in modern English, particularly when referring to a romantic partner.