litotes

 
Pronunciation: /lʌɪˈtəʊtiːz/

noun

[mass noun]
  • ironical understatement in which an affirmative is expressed by the negative of its contrary (e.g. I shan’t be sorry for I shall be glad).

Origin:

late 16th century: via late Latin from Greek litotēs, from litos 'plain, meagre'