Base 8. A number representation using 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 system used to be widespread back when many computers used 6-bit bytes, as a 6-bit byte can be conveniently written as a two-digit octal number. Since nowadays a byte is almost always 8-bit long the octal system lost most of its appeal to the hexadecimal system.

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