Common name for a kind of blood collection set used in most phlebotomy clinics. It consists of a small-gauge hypodermic needle embedded in a piece of plastic with two "wings" branching out that allow the technician to hold the device. The needle feeds into a ~10cm flexible tube that is then poked into the vacuum vial for collection.

Butterflies are usually used for blood draws from the lower arm and wrist. I prefer to call them "mosquito"s, as the name fits much better, but humorless phlebotomists might not understand what you are talking about when you request for them to use one, as I found out to my chagrin on my last doctor's visit.