The term "one-off" is a common term in the UK, Australia and New Zealand. It's also commonly used in the US or Canada, though to a lesser extent.
There are mainly 3 types of "one-off"s in English.
- 一次性 (Formal, sometimes governmental) activities done, made, or happening only once. When it ends it will not be done / be created / happen again.
one-off donation 一次性捐款 /
one-off grant 一次性拨款 /
one-off deduction 一次性扣除
Chile made a one-off cash bonus payment to the poorest 40 per cent.
Government introduces one-off resident visa for 160,000 migrants
In some Chinese-speaking communities where 性 is used exclusively for sex-related terms in local Chinese varieties, it's translated differently. In Hong Kong: 一次過.
- No corresponding term in Chinese. Events / things done, made, or happening only once. Not counted as part of a regular sequence.
The concept of emergency as a one-off, unpredicted crisis also needs to be reconsidered.
(Technically speaking, "偶发" is not the equivalence of "one-off". )
The meeting is a one-off. 这个会议是临时的。
(Technically speaking, one-off meeting ≠ ad hoc meeting 临时（特设）的会议. )
- 只/仅此一* (* = a measure word，e.g. 只此一次、只此一家、只此一件) Something that is unique and oftentimes valuable. If you miss it you'll never see / use / experience it again. You will regret it.
New Ferrari SP51 unveiled as a one-off commission
法拉利新车SP51正式亮相，全球仅此一辆 / 仅此一台
Blur Reuniting For A ONE-OFF Show At Wembley Stadium
There are mainly 3 types of "一次性"s in Chinese.
一次性手套 disposable gloves
一次性捐款 one-off donation
- In one go
一次性通过所有考试 pass all exams in one go