Understatement.
[n] understatement for rhetorical effect (especially when expressing an affirmative by negating its contrary); "saying `I was not a little upset' when you mean `I was very upset' is an example of litotes"
[n] (genetics) cell division that produces reproductive cells in sexually reproducing organisms; the nucleus divides into four nuclei each containing half the chromosome number (leading to gametes in animals and spores in plants)