Tessellation
Tessellation is a pattern of shapes that fit together with no gaps or overlaps.
Think of it this way
Bathroom floor tiles fit together like a perfect puzzle. Each square tile touches its neighbors on all four sides. No space is left uncovered, and no two tiles overlap. That tight, gap-free fit is exactly what tessellation means.
Explaining tessellation by grade level
Look at a honeycomb. Each six-sided shape fits next to the others. There are no gaps between them. That snug fit is a tessellation.