Consisting of eight members or of groups containing eight.(adjective)
A group or set of eight.(noun)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "octonary" in a sentence
"That which many do comment concerning the octonary number of beatitudes hath too much curiosity, and little benefit."
"But such men experience no difficulty whatever in performing their mental computations in the decimal system; and they acquire through constant practice such quickness and accuracy of calculation, that it is difficult to see how octonary reckoning would materially assist them."
"It is said that the ancient Saxons used the octonary system,  but how, or for what purposes, is not stated."