semicircular canal

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of three tubular and looped structures of the inner ear, together functioning in maintenance of the sense of balance in the body.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun one of three tube loops filled with fluid and in planes nearly at right angles with one another; concerned with equilibrium