In base eight (5)
I believe the answer is:
octal
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">in base eight</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)