octōgōnŏs

i, adj.
I. eight-cornered, octagonal: turris marmorea octogonos, Vitr. 1, 6, 4.— As subst.: octōgōnum, i, n., an octagon, Vitr. 1, 6, 4 al.
Lewis & Short
A Latin Dictionary, 1879
An open-access project