Of base eight (5)
I believe the answer is:
octal
'of base eight' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for octal that I've seen before include "Working in base 8" , "based on ...[EIGHT]" , "with unusual base" , "Pertaining to a number system with 8 as its base" , "type of base" .)