Its officially called Tribadism but it also has a number of colloquial terms, some of them quite comical. Heres some of my favs:
Battle at the Red Bridges
Beaver Bumping
Biscuit Waxing
Cracker Barreling
Cut Melon Smashing
French Connection
Gash Mashing
Hamster Grappling
Muff Buffing
Pocket Docket
Rose-Bud-Thud
Touching Tacos
Tuna Taco Tango
can't really imagine any of them being used in dirty talk though...