Litotes

Stylistic Devices – Litotes

form of understatement

Litotes is a form of understatement which uses the denied opposite of a word to weaken or soften a message.

Examples:

* That's not bad. (instead of: That's good/great.)
* Boats aren't easy to find in the dark. (4) (instead of: Boats are hard/difficult to find in the dark.)