An apse is the curved inside end of a building — most often the east end of a church, where the wall bows out in a half-circle (sometimes in a many-sided shape that still feels round). If you stand in the main hall and look toward the altar, the apse is often that pocket of space behind or around it, with a curved ceiling like a shallow round roof. From outside, you can sometimes see the same curve sticking out of the back of the building. Other grand buildings can have an apse too, but church buildings are where students will meet the word. It belongs to architecture and guidebooks; it is not a word for ordinary rooms at home.