one-off (noun)

1. Their surprise trip to Paris was a one-off romantic gesture that they would always cherish.

2. The unicorn sighting in the forest was a one-off event that was believed to be a result of a magical spell.

3. The creation of an artificial intelligence with emotions was considered a one-off scientific experiment.

4. The author's unpublished manuscript was a one-off masterpiece that was discovered after their death.