Hexadecimal notation that is used to define a byte (i.e., eight bits) of information, where a and b each specify four bits of information and have values from the range {0, 1, 2,…F}. For example, 0xc6 is used to represent 11000110, where c is 1100, and 6 is 0110.
Sources:
NIST SP 800-90A Rev. 1