In computing, a nybble, aka “nibble,” is a set of four bits. Since there are eight bits in a byte, a nybble is half of one byte. In the computer world, two nybbles always equal one byte. The four bits in a nibble allow it to have 16 possible values, which is the same as one hexadecimal digit. Therefore, a nybble is sometimes referred to as a “hex digit.” In data communications, nybbles are sometimes called “quadbits,” because of the four bits that make up each nybble.
The term originates from the term “byte” which is a homophone of the English word “bite”. A nibble is a small bite, which in this context is construed as “half a bite”. Early use of the term “nybble” occurred in 1977 by the technology group at Citibank that created a standard for transactional messages, between cash machines and Citibank’s data centers, in which a nybble was the basic informational unit. (wiki, Tech Terms)