The acronym N.O.S stands for 'new old stock' and the word can be used for the kind of spare parts that are not in production but available in stock. The following explanation can make you understand the word well: suppose you have a bike that is old and the company producing the bike has stopped the production of that model. If you find some problem in your bike and if it requires changing a spare part but as the bike is out of production the spare part might also be out of production. In that case if you contact the company or an old dealer who can give you the part from the old stock. In this case the part has never been used so it is new but it is not in production so you can call it old also. The term N.O.S is used for this kind of product.