When a part is described as "in NOS" condition, what does it mean?

NOS stands for New-Old Stock. In other words, it is an original part that was never sold. It may have been in a warehouse or in a dealerships inventory. Some parts are non-returnable to the factory when a model is discontinued and the dealer is left with trying to get rid of the part. It will be from the original prduction run of replacement parts, ie. not a reproduction of an original part from an aftermarket supplier.

NOS parts are in high demand, not only with Z's but any collectible car. Companies will go out and search for dealerships that are going out of business, or just trying to unload excessive inventory of parts, and this is how NOS parts are brought to the market at a somewhat inflated price. It is especially prevelent in the US muscle car business, where a lot of the parts are date coded.

Could also be used as a short for Nitrous Oxide. Depends on where you are having this discussion.