Octa means eight. An octagon is an 8-sided polygon. A regular
octagon has all its sides of equal length and all its angles
equal to 135^{o}.

A regular octagon is an octagon which has all same length
sides and whose internal angles are all the same size. The
internal angle at each vertex of a regular octagon is 135^{o}
and the sum of all the internal angles is 1080^{o}.