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 errors, though it cannot eliminate them entirely.

--The Jargon File version 4.3.1, ed. ESR, autonoded by rescdsk.

Zeroth is the ordinal number that precedes first. Since zero is not a counting number, when people create lists, they tend to call the initial item the "first". So you get the First Amendment, the First Law of Thermodynamics, etc.

In most cases, when people make additions to the list, they will pick the next ordinal number (second, third, fourth, etc.). However, sometimes this additional item is found to be more fundamental than the "first" item was. It's impractical to call this new one "first" and renumber the existing ones, so the new, fundamental list item is called the "zeroth" item. The two main uses of "zeroth" in this context are the Zeroth Law of Robotics and the Zeroth Law of Thermodynamics.