octo

Octo is a combining form meaning eight. It appears in English in many words, from the Latin prefix octo- to the Greek form okto-, and it denotes the number eight. The spelling can vary as octo- or octa- depending on linguistic context and the word it forms.

In science and mathematics, octo- is used to form names such as octagon (an eight-sided polygon), octahedron (an eight-faced polyhedron), octet (a group of eight), and octane (a hydrocarbon with eight carbon atoms). The word octopus, while common in everyday language, derives from Greek okto- and pous, meaning eight feet or arms; octopuses are classified in the order Octopoda.

In biology, octopus is the best-known example of an eight-armed animal, reflecting the root meaning. In music, octave denotes eight notes of the same pitch class, illustrating how the concept of eight spans multiple domains. The prefix also appears in terms like octogenarian (a person in their eighties) and in October, which originally referred to the eighth month of the ancient Roman calendar.

In modern technology and culture, octo- appears in phrases such as octo-core processors and in octothorpe, the formal name for the symbol '#', a term sometimes encountered in puzzle communities.

Overall, octo serves as a productive root across fields to signal the number eight, with spelling variations such as octo- or octa- reflecting its linguistic origins.