NOS means "New Old Stock" (and also Nitrous Oxide Systems which is a company that makes performance auto parts).
The NOS acronym is almost always used, you rarely ever see the words "New Old Stock" in print, although it is commonly used in actual conversation.
NOS most commonly refers to old replacement or repair parts that have never been used. It does not however refer to entire units that are unused. For example you can have an NOS steering wheel for a 1966 Mustang, or an NOS monitor bezel for a Ms. Pac-Man machine. But you cannot have an NOS 1966 Mustang, or an NOS Ms. Pac-Man (the phrase usually does not extend to complete units, even if they are unused, when it is used for complete units, they are most often small, like bicycles).
NOS parts may exist for anything that could possibly need repair parts, but they are most commonly available for items that were either very popular, or were meant to be popular, but never quite caught on. For example, NOS parts for old arcade games like Pac-Man and Robotron 2084 are still easy to locate because they were popular games, and the manufacturers overproduced the parts. But NOS parts for less popular titles such as SHARK Jaws and Arlington Horse Racing are non-existent because there was no demand for them back when the manufacturers still supported those titles. Then there are the cases where the availability of parts wildly outnumbers the actual availability of units to install them into. This is the case with the game Pac-Man Plus, there are literally more NOS marquees (nameplates), laying around for this game, than there are machines to install them into.
A part has to be out of production for several years before the NOS term can truly apply. For example, they still make headlight assemblies for the early 90s Chevrolet trucks, so the NOS term doesn't apply.
Reproduction parts are different from NOS parts. They are new parts made to mimic the form and functionality of the original (out of production) parts for an item. Reproduction parts are sometimes exactly the same as the original, and may even have improvements. But collectors (the people who are most concerned about NOS parts), tend to prefer original parts, even if the new ones are better.
You might ask yourself, "If there are all these old parts laying around, then how come the auto parts store never has the stuff I need for my 1976 Ford Pinto?". The answer to that is simple, retailers, distributors, and repair shops generally stop carrying parts for any given item once the demand drops below a certain level. Those old parts eventually end up in a warehouse, where they might sit forgotten for many years, or even decades. Then eventually they will be noticed again (at which time there is usually a demand for the parts again). It probably goes something like this.
"Hey Earl, what is in all those dusty boxes over in the corner?"
Beats me Frank, they have been sitting there as long as I can remember, go open one of them up and look.
Then the boxes will be opened, and all those old jukebox parts, auto parts, or whatever they are will be discovered again, and will eventually make their way into the hands of someone who will start distributing them again.