If you want to sound a little old-fashioned, you might refer to your girlfriend as your inamorata.
A woman you love in a romantic way is your inamorata, although it's not a very common term these days. It would be a little startling if your friend said, "I'd like to introduce you to my inamorata," though you'd know he meant "the woman I love." The male version of an inamorata is inamorato, which is even more rarely used. Both words come from the Italian innamorare, "to fall in love."