A tube top is a shoulderless, sleeveless "tube" that wraps the torso (not reaching higher than the armpits). Such a top is generally very tight over the breasts in order to prevent the garment from falling. Usually this is prevented with elastic.
A similar version is one model of the halter top, which is a tube top that uses a single strap that passes round the back of the neck and has both ends attached to the front of the tube. Third wave ska band Reel Big Fish released a song entitled "Ban the Tube Top" in which they bemoaned the style.
