WordNet (r) 3.0 (2006):

nybble
    n 1: a small byte [syn: nybble, nibble]

The Jargon File (version 4.4.7, 29 Dec 2003):

nybble
 /nib'l/, nibble, n.

    [from v. nibble by analogy with ?bite? ? ?byte?] Four bits; one hex
    digit; a half-byte. Though ?byte? is now techspeak, this useful relative is
    still jargon. Compare byte; see also bit. The more mundane spelling ?
    nibble? is also commonly used. Apparently the ?nybble? spelling is uncommon
    in Commonwealth Hackish, as British orthography would suggest the
    pronunciation /ni:'bl/.

    Following ?bit?, ?byte? and ?nybble? there have been quite a few analogical
    attempts to construct unambiguous terms for bit blocks of other sizes. All
    of these are strictly jargon, not techspeak, and not very common jargon at
    that (most hackers would recognize them in context but not use them
    spontaneously). We collect them here for reference together with the
    ambiguous techspeak terms ?word?, ?half-word?, ?double word?, and ?quad? or
    quad word; some (indicated) have substantial information separate entries.

    +-------------------------------------------------------------------------+
    |2     |crumb, quad, quarter, tayste, tydbit, morsel                |
    |bits: |                                                                  |
    |------+------------------------------------------------------------------|
    |4     |nybble                                                            |
    |bits: |                                                                  |
    |------+------------------------------------------------------------------|
    |5     |nickle                                                          |
    |bits: |                                                                  |
    |------+------------------------------------------------------------------|
    |10    |deckle                                                          |
    |bits: |                                                                  |
    |------+------------------------------------------------------------------|
    |16    |playte, chawmp (on a 32-bit machine), word (on a 16-bit         |
    |bits: |machine), half-word (on a 32-bit machine).                        |
    |------+------------------------------------------------------------------|
    |18    |chawmp (on a 36-bit machine), half-word (on a 36-bit machine)   |
    |bits: |                                                                  |
    |------+------------------------------------------------------------------|
    |32    |dynner, gawble (on a 32-bit machine), word (on a 32-bit         |
    |bits: |machine), longword (on a 16-bit machine).                         |
    |------+------------------------------------------------------------------|
    |36    |word (on a 36-bit machine)                                        |
    |bits: |                                                                  |
    |------+------------------------------------------------------------------|
    |48    |gawble (under circumstances that remain obscure)                |
    |bits: |                                                                  |
    |------+------------------------------------------------------------------|
    |64    |double word (on a 32-bit machine) quad (on a 16-bit machine)      |
    |bits: |                                                                  |
    |------+------------------------------------------------------------------|
    |128   |quad (on a 32-bit machine)                                        |
    |bits: |                                                                  |
    +-------------------------------------------------------------------------+

    The fundamental motivation for most of these jargon terms (aside from the
    normal hackerly enjoyment of punning wordplay) is the extreme ambiguity of
    the term word and its derivatives.


The Free On-line Dictionary of Computing (30 December 2018):

nibble
nybble

    /nib'l/ (US "nybble", by analogy with "bite" -> "byte")
   Half a byte.  Since a byte is nearly always eight bits, a
   nibble is nearly always four bits (and can therefore be
   represented by one hex digit).

   Other size nibbles have existed, for example the BBC
   Microcomputer disk file system used eleven bit sector numbers
   which were described as one byte (eight bits) and a nibble
   (three bits).

   Compare crumb, tayste, dynner; see also bit, nickle,
   deckle.

   The spelling "nybble" is uncommon in Commonwealth Hackish as
   British orthography suggests the pronunciation /ni:'bl/.

   (1997-12-03)