In pre-calculus, the unit circle is sort of like unit streets, it’s the very small circle on a graph that encompasses the coordinates. It has a radius of hence the unit. The figure here shows all the measurements of the unit circle:
Phylogenetic Tree of Life -The OpenLearn site used to offer this phylogenetic chart in the shape of a tree as a free poster, but no more. They do have an online version of this same illustration, expandable and with links and an explanatory guide.