Noun
A diminution or softening of statement for the sake of avoiding censure or increasing the effect by contrast with the moderation shown in the form of expression; as, " a citizen of no mean city," that is, of an illustrious city.
Source: Webster's dictionaryPreachers at black churches are the last people left in the English-speaking world who know the schemes and tropes of classical rhetoric: parallelism, antithesis, epistrophe, synecdoche, metonymy, periphrasis, litotes - the whole bag of tricks. P. J. O'Rourke
saying `I was not a little upset' when you mean `I was very upset' is an example of litotes Source: Internet