Definition of "antrum" [an•trum]

• A cavity or chamber, especially one in a bone. (noun)
• Either of the sinuses in the bones of the upper jaw, opening into the nasal cavity. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "It is somewhat conical with its base applied to the upper orifice of the stomach, and is known as the antrum cardiacum."
• "The entrance to the antrum is a large irregular aperture, which leads backward from the epitympanic recess into a considerable air space, named the tympanic or mastoid antrum (see page 142)."
• "On the medial wall of the entrance to the antrum is a rounded eminence, situated above and behind the prominence of the facial canal; it corresponds with the position of the ampullated ends of the superior and lateral semicircular canals."