Related to numbers:
* Octagon: A polygon with eight sides.
* Octal: A number system with base 8.
* Octave: An interval in music encompassing eight notes.
* Octuple: Eight times as much.
* Octogenarian: A person between 80 and 89 years old.
Other words:
* October: The tenth month of the year.
* Octopus: A cephalopod mollusk with eight arms.
* Octroi: A tax levied on goods entering a city or town.
* Octopod: Having eight feet or arms.
* Octant: A navigation instrument for measuring angles.
Less common words:
* Octodecimo: A book size with pages folded eight times.
* Octophonic: Having eight sound channels.
* Octarchy: Government by eight people.
* Octosyllabic: Consisting of eight syllables.
* Octogen: An eight-sided pyramid.
* Octometer: A line of poetry with eight metrical feet.