I was introduced to the term one-off with a slightly different meaning than the one Grant gave. I interned at a company that made small unmanned aircraft, and the complete ground control setup. There, I recall using the term one-off to refer to a product that has been modified from the standard configuration, usually to suit the needs of a particular customer, with no intention of it becoming a new product. Occasionally, of course, they would realize that the idea was marketable and make it into a standard product, but not always. These weren't completely custom, so much as a system that might have one or two custom components that integrated into the standard system.

As I understood it, the distinction was important because engineering costs for standard products was overhead, and wasn't (directly) passed on to the customer, but the one-off costs were billed directly to the customer requesting them. Also, there were many quality processes for standard products, but if something was declared a one-off, the requirements were much more relaxed.

With that usage, I'd assumed that it referred to an item/product that had one feature/aspect/component that was different than the standard, or "off of" it.