antrum

noun, plural an·tra [an-truh] /ˈæn trə/. Anatomy.

1. a cavity in a body organ, especially a bone.

noun plural -tra (-trə)

1. anatomy a natural cavity, hollow, or sinus, esp in a bone

n. pl. an•tra (-trə)

1. A nearly closed cavity or chamber, especially in a bone.
2. The pyloric end of the stomach, partially shut off during digestion from the cardiac end by sphincter muscles in the stomach wall.