Nibble

A four-bit aggregation (half a byte or half an octet) that can represent the decimal values 0 to 15 (16 distinct values). Since there are 16 possible values, a nibble corresponds to a single hexadecimal digit and is sometimes referred to as a "hex digit" or "hexit".

The following is a 4-bit nibble:

1 0 0 1

Source: Wikipedia