Hexadecimal is a number writing system that uses a base of 16. It uses 16 distinct symbols such as "0" - "9" to represent 0 to 9, and "A" - "F" to represent values 10 to 15. Each hexadecimal digit represents four bits, also known as a nibble, which is 1/2 of a byte.

In C the prefix 0x is used to denote hexadecimal numbers. An example being 0x00 to represent 00000000 in binary.