For example, every hexadecimal digit has an equivalent 4 digit binary number.
From the en.wikipedia.org
However, when mapping to hexadecimal, it is trivial to regard the binary string as 4-digit groups and map each to a single hexadecimal digit.
From the en.wikipedia.org
The nibble is used to describe the amount of memory used to store a digit of a number stored in binary-coded decimal format, or to represent a single hexadecimal digit.
From the en.wikipedia.org
The asterisk is immediately followed by a checksum represented as a two-digit hexadecimal number.
From the en.wikipedia.org
One nibble corresponds to one digit in hexadecimal and holds one digit or a sign code in binary-coded decimal.