litote
A litote is an understatement that expresses the affirmative by saying the negative of the contrary. This is a wordy definition that can make it difficult to understand. Here are a few examples:
That wasn't the messiest house I've ever seen. - This sentence says that the house is messy by saying the contrary - that it's not messy.
The poem "Fire and Ice" by Robert Frost has a litote because it says that the world being destroyed would be "great" and "would suffice". He doesn't actually think this to be true.