hexadecimal 
While the more common decimal system uses a base of 10 to represent all possible numbers, hexadecimal notation uses a base of 16: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, A, B, C, D, E, F. While such a system may seem confusing at first, it works extremely well in computing environments. For example, a single byte of information can be represented as eight bits (10011101), decimal numerals (913), or simplified to hex (9D). It's easier to read two hexadecimal characters than 10011101.

As it relates to Web design, hexadecimal is the alphanumeric system used to specify colors in HTML. For example, the hexadecimal equivalent of white is FFFFFF, while black is 000000.