Meaning of “one-off” - Learner’s Dictionary

one-off

adjective [ always before noun ] uk /ˌwʌnˈɒf/ UK

only happening once:

a one-off payment
one-off noun [ C ] UK

something that only happens once:

His Olympic victory was not just a one-off.

(Definition of “one-off” from the Cambridge Learner’s Dictionary © Cambridge University Press)