NOS stands for:
- New Old Stock: A part which is unused but not brand new. Usually it refers to a geunine factory or dealer part which is NLA (No Longer Available) but someone has said part and is offering it for sale. However, it can also be applied to aftermarket parts, and parts made in recent time. Basically it means the part is not brand-spanking new, but is as-new unused.
- Nitrous Oxide Systems: A brand manufacturer of nitrous oxide (N2O)engine performace parts.