1) n. In New England, it is a brass musical instument, conical in construction, about 18 feet long, with a large mouthpiece, and is the lowest of the brass instruments. It replaced the ophiclede in the symphony orchestra, and has nothing to do with Wagner Tubas. It has only one direct cousin in brass instruments, that being the fluglehorn, for all other brass instruments are more cylindrical than the tuber, even the french horn.
2) n. For the rest of the English-speaking world, it is a fleshy-rooted plant like a potato or a yam. Hence, New Englanders and Non-New Englanders can get confused at times, as evidenced below.
An actual conversation:
Denny: Dan, can you play the tuber for the Bristol, RI band next weekend?
Dan: Well Denny, do you want me to play 1st or 2nd potato?