From WikiChip

A nibble (often nybble or nyble) is 4-bits of data. It may also be called a half-byte, quadbit, quartet, semi-octet, or tetrade. A nibble has 16 (24) possible values. Since a hexadecimal digit is also able to represent up to 16 distinct values, it is common for a nibble to be represented as a single hexadecimal. Four bit architectures use the nibble as their must fundamental unit.