Mosaic is the art of creating images with an assemblage of small pieces of colored glass, stone, or other materials. It may be a technique of decorative art, an aspect of interior decoration, or of cultural and spiritual significance as in a cathedral. Small pieces, mostly roughly square, of stone or glass of different colors, known as tesserae, (diminutive tessellae), are used to create a pattern or picture.
In architecture, the apse (Greek ἀψίς (apsis), then Latinabsis: "arch, vault"; sometimes written apsis; plural apses) is a semicircular recess covered with a hemispherical vault or semi-dome, also known as an Exedra. In Romanesque, Byzantine and Gothic Christian abbey, cathedral and churcharchitecture, 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.