means The arrangement of shapes that fit together perfectly with no gaps or overlaps, repeating to cover a surface — like tiles on a floor or the cells of a honeycomb.

from From Latin 'tessella,' a small cube or square tile used in mosaics, itself a diminutive of 'tessera,' the little four-sided piece. 'Tessera' likely traces back to Greek 'tessares,' meaning four — fittingly, since the humblest tile has four sides. Roman floors were paved with thousands of these tesserae, so the word for fitting tiny squares together neatly was already ancient before mathematicians borrowed it to describe the geometry of the pattern itself.