WordNet (r) 3.0 (2006):

zeroth
    adj 1: preceding even the first

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

zeroth
 /zee'rohth/, adj.

    First. Among software designers, comes from C's and LISP's 0-based indexing
    of arrays. Hardware people also tend to start counting at 0 instead of 1;
    this is natural since, e.g., the 256 states of 8 bits correspond to the
    binary numbers 0, 1, ..., 255 and the digital devices known as counters
    count in this way.

    Hackers and computer scientists often like to call the first chapter of a
    publication ?Chapter 0?, especially if it is of an introductory nature (one
    of the classic instances was in the First Edition of K&R). In recent
    years this trait has also been observed among many pure mathematicians (who
    have an independent tradition of numbering from 0). Zero-based numbering
    tends to reduce fencepost errors, though it cannot eliminate them
    entirely.


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

zeroth

    First.

   Since zero is the lowest value of an unsigned integer,
   which is one of the most fundamental types in programming and
   hardware design, it is often natural to count from zero
   rather than one, especially when the integer is actually an
   index or offset, as used when addressing hardware and
   arrays.

   Hackers, computer scientists and pure mathematicians often
   like to call the first chapter of a publication "Chapter 0",
   especially if it is of an introductory nature (one of the
   classic instances was in the First Edition of K&R).

   Zero-based numbering tends to reduce fencepost errors,
   though it cannot eliminate them entirely.

   Logically, the next item after the zeroth should be the
   "oneth" but this is never used.

   [Dijkstra, "Why Numbering Should Start at Zero"

(http://www.cs.utexas.edu/users/EWD/transcriptions/EWD08xx/EWD831.html)].

   [Jargon File]

   (2010-02-28)