In base eight (5)
I believe the answer is:
octal
'in base 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 "Working in base 8" , "with unusual base" , "Relating to the number 8" , "Pertaining to a number system with 8 as its base" , "(Of maths) in base eight" .)