a girl or young woman with whom a man is romantically involved
"his girlfriend kicked him out"
A female partner in an unmarried romantic relationship.
"Sarah and Mark have been dating for two years, making her his girlfriend even though they are not married yet."
In plain English: A girlfriend is a girl you are dating and have a romantic relationship with.
"She invited her girlfriend over for dinner tonight."
Usage: Use girlfriend to refer specifically to your current, exclusive romantic partner rather than just any friend or acquaintance you spend time with. Avoid using it as a generic term for all women unless the specific woman has been explicitly identified as your partner.
Girlfriend is a compound word formed by combining "girl" and "friend." It originally meant simply a female companion or associate.