Source : WordNet®

zeroth
     adj : preceding even the first

Source : Free On-Line Dictionary of Computing

zeroth
     
         First.
     
        Since zero is the lowest value of an {unsigned} {binary}
        {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}, as in hardware addressing or {C} and
        {Lisp}'s 0-based indexing of {arrays}.
     
        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.
     
        Logically, the next item after the zeroth should be the
        "oneth" but this is never used.
     
        [{Jargon File}]
     
        (1997-12-07)