A mosaic is a pattern or image made of small pieces without gaps or overlaps. People have made mosaics for thousands of years. They appear in ancient Greece, medieval churches, Islamic palaces and modern public art.
We think of mosaic using stone or glass pieces but it can be made from small pieces of any material that fit tightly together. It can be squares or complicated shapes. Examples are found in tiled floors, patchworks, embroidery, puzzles and digital art. You can see something similar in nature, for example the hexagonal shapes of the honeycomb in a beehive. The idea in mathematics is called tiling or tessellation (from the Latin word tessera for a square of stone used in Roman mosaics).