A double made in the belief that the opposing contract will not make. Another name for this is a ‘business double’.
For example:
![](https://bernardmageebridge.com/wp-content/uploads/2021/11/image-207.png)
West’s double of 1NT is a penalty double, as is East’s double of North’s retreat to 2♠.
Whether a double is a penalty double is partly a matter of general bridge knowledge and partly a matter of partnership agreement.