A unique and distinct example of an established form. aka "one-shot". almost always physical objects, often custom-made, frequently seen as highly collectable or desirable due to their individuality. sometimes used to describe an event, such as a music concert.
Primarily used as an adjective, may also be used as a noun.
examples: concept cars. custom jewelry or clothes. especially when made by producers of high-quality mass-produced items such as furniture or electronics.

sometimes used to refer to a recordable cd, or "CD-R".
used in the phrase "one-off billet" to describe custom machine parts.