Based on eight (5)
I believe the answer is:
octal
'based on eight' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for octal that I've seen before include "Of a number system with base eight" , "(Of maths) in base eight" , "Working in base 8" , "Relating to the number 8" , "Using 8 as base (maths)" .)