WordNet (r) 3.0 (2006):

octal
    adj 1: of or pertaining to a number system having 8 as its base;
           "an octal digit"

The Free On-line Dictionary of Computing (30 December 2018):

octal

    Number base eight.  The octal number
   representation uses the digits 0-7 only, with the right-most
   digit counting ones, the next counting multiples of 8, then
   8^2 = 64, etc.  For example, octal 177 is digital 127:

   	digit    weight        value
   	  1     8^2 = 64   1* 64 = 64
   	  7     8^1 =  8   7*  8 = 56
   	  7     8^0 =  1   7*  1 =  7
   			          ---
   				  127

   Octal representation used to be widespread back when many
   computers used six-bit bytes, as six-bits can be
   conveniently written as a two-digit octal number.  Since
   nowadays a byte is almost always eight bits long, the octal
   system lost most of its appeal to the hexadecimal system.
   Octal is still found in the C programming language and its
   descendents where it is commonly to represent characters, as
   in 'A' = '\101', 101 being octal for 65, the ASCII
   character code for 'A'.

   For a brief discussion on the word `octal' see hexadecimal.

   (2017-12-18)