Meiosis is a rhetorical term which comes from the Greek ‘to diminish’.
Meiosis occurs when someone says less than they really mean.
Litotes is a particular type of understatement, and is therefore a type of meiosis. In order for an understatement to be ‘litotes’, an affirmative is expressed by negating its opposite.
Another type of meiosis is ‘tapiosis’ which is the literary term for ‘name calling by belittling someone’.