Dictionary.com explains a tessellation to be a tessellated form or arrangement.
Math Forum. org describes a tessellation to be when a shape is repeated over and over again covering a plane without any gaps or overlap.
Both of these are true however, I believe a tessellation to be much more than that.
Tessellations are used in everyday life.
We may not notice them but somehow we still appreciate them, each in our own way.
Here are some tessellations which I created my self.
They shows no gaps or overlap while being repeated across the page.