A tessellation is created when a shape is repeated over and over again covering a plane without any gaps or overlaps.
Another word for a tessellation is a tiling. Read more here: What is a Tiling?
A dictionary* will tell you that the word “tessellate” means to form or arrange small squares in a checkered or mosaic pattern. The word “tessellate” is derived from the Ionic version of the Greek word “tesseres,” which in English means “four.” The first tilings were made from square tiles.
Information around the math and theory behind tessellation patterns: