A number system, unlike the one we are used to using, that is based on 16. Letters are used to signify the numbers 10-15, so that counting goes thusly: 1 2 3 4 5 6 7 8 9 A B C D E F. Then 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 1A, 1B, 1C, 1D, 1E, 1F, 20, etc. Conventionally "0x" is placed in front of a hexadecimal number: 0x1F, 0xAD, etc.