Understatement

UN-der-state-ment

The opposite of exaggeration. Strangely, an understatement can sometimes amplify your point better than exaggerating it would.

See litotes. 

Nothing in life is so exhilarating as to be shot at without result. Winston Churchill

Our ships have been salvaged and are retiring at high speed toward the Japanese fleet.  Admiral William Halsey in 1944, after the Japanese claimed that the American Third Fleet had been sunk or had “retired.”