The main part of either auricle of the heart as distinct from the auricular appendix. Also, the whole articular portion of the heart.

a chamber or cavity (of the heart, etc.).

Either one of the heart's two upper chambers.

a heart chamber that receives blood from the veins; plural: atria

A square hall lighted from above, into which rooms open at one or more levels.

An open court with a porch or gallery around three or more sides; especially at the entrance of a basilica or other church. The name was extended in the Middle Ages to the open churchyard or cemetery.

A cavity in ascidians into which the intestine and generative ducts open, and which also receives the water from the gills. See Ascidioidea.

A cavity, entrance, or passage; as, the atrium, or atrial cavity, in the body wall of the amphioxus; an atrium of the infundibula of the lungs, etc.

An enclosed glazed space, normally in the middle of buildings of 3 storeys or more, which allows light into the centre and traps heat.

an enclosed space in an animal body.

In ctenostomes; space between the tips of the tentacles and the orifice, in a retracted zooid (Hayward, 1985). In stenolaemates, space between tips of the tentacles (when withdrawn) and the orifice of the autozooid, sealed distally and proximally by sphincter muscles (Hayward & Ryland, 1985; but see Boardman, 1998).