The term "one-time" refers to something that happens only once or a single occurrence. It can also be used as an adjective to describe a one-off event, action or item - i.e., something that is unique and not repeated.
Usage examples
We offer a onetime discount for new customers.
The onetime event was a huge success, drawing a large crowd.
He made a onetime donation to the charity.
I went on a onetime adventure and traveled to a remote island.
They had a onetime opportunity to meet their favorite celebrity.