What is "octa-"

Octo- \Oc"to-\, Octa- \Oc"ta-\ [L. octo eight, Gr. 'oktw`, with a combining form 'okta-. Cf. Eight.] A combining form meaning eight; as in octodecimal, octolocular.

Octa- \Oc"ta-\ A prefix meaning eight. See Octo-.

before vowels oct-, word-forming element meaning "eight," from Greek okta-, okt-, from PIE *okto(u) "eight" (see eight). The variant form octo- often appears in words taken from Latin, but the Greek form is said to be the more common in English.

pre. eight