litotes
1. The affirming of a thing by denying its contrary.
His philology is not irresponsible.
Her statement is not inaccurate.
Her statement is not inaccurate.
litote
Understatement. When you say something (usually satirically) by saying the opposite of what you really mean. Like "That's not good." When you mean "Holy crap, that sucks! What are we going to do!?"
I didn't want to offend the teacher, so I used litote to tell her how much her class sucks.