zeroth: /zee'rohth/ adj. First. Among software designers,

: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.
-- The AI Hackers Dictionary