Octo is a prefix meaning eight, borrowed from Latin. You'll find it at the beginning of many English words related to the number eight.

An octopus has eight arms. An octagon is a shape with eight sides, like a stop sign. October was originally the eighth month in the ancient Roman calendar, before January and February were added to the beginning of the year. When musicians talk about an octave, they mean a distance of eight notes on a musical scale.

The prefix shows up in less common words too: an octogenarian is someone in their eighties, and octuplets are eight babies born at the same time.

You might also see oct or octa used the same way, as in octet (a group of eight musicians) or octahedron (a three-dimensional shape with eight faces).