None really, the definition of an apse changed over time and geography not religions. According to Wikipedia: http://en.wikipedia.org/wiki/Apse .
In architecture, th…e apse (Greek Î±ÏÎ¹Ï (apsis), then Latin absis : "arch, vault"; sometimes written apsis ; plural apses ) is a semicircular recess covered with a hemispherical vault or semi-dome. In Romanesque, Byzantine and Gothic Christian abbey, cathedral and church architecture, the term is applied to a semi-circular or polygonal termination of the main building at the liturgical east end (where the altar is), regardless of the shape of the roof, which may be flat, sloping, domed, or hemispherical. . ( Full Answer )