A brand of flavored vodkas. Though the line includes UV Green (Apple), UV Red (Cherry), UV Ivory (Vanilla), and UV Orange (Orange), the most popular is UV Blue, which is Blue Raspberry flavored. UV is good for mixing, but is often taken as shots by sorority chicks and other lightweight drinkers. Very popular at college parties, shots are often sold for $1.
At the kegger, Sara didn't like beer, so she paid a dollar and got a shot of UV.
Since UV 1 is just a brand name, while it may be popular in certain locations, I don't think it's the definitive definition. Never heard UV 2 used before either (maybe a locality thing too but not as likely as UV 1) so I am going with the standard... UV = Ultra Violet as in light wave. Especially referring to lights.
Man, that babe's white tee-shirt lit up like the moon when she walked under the uv lights at the gig!