zeroth

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 error}s, though it cannot eliminate them
   entirely.