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.-the main part of either auricle of the heart as distinct from the auricular appendix. also, the whole articular portion of the heart.-a cavity in ascidians into which the intestine and generative ducts open, and which also receives the water from the gills. see ascidioidea.