display | more...

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.