Pronunciation: (ā'trē-um),[key] —n., —pl.a•tri•aPronunciation: (ā'trē-u),[key]a•tri•ums. 1. Archit. a. Also calledcavaedium. the main or central room of an ancient Roman house, open to the sky at the center and usually having a pool for the collection of rain water.
b. a courtyard, flanked or surrounded by porticoes, in front of an early or medieval Christian church. See diag. under basilica. c. a skylit central court in a contemporary building or house.
2. Anat.either of the two upper chambers on each side of the heart that receive blood from the veins and in turn force it into the ventricles. See illus. under heart.