understatement

noun

1. the act or an instance of understating, or representing in a weak or restrained way that is not borne out by the facts: The journalist wrote that the earthquake had caused some damage. This turned out to be a massive understatement of the devastation.

noun

1. the act or an instance of stating something in restrained terms, or as less than it is

n.1799, from under + statement. Understated, with reference to clothing, is recorded from 1957. A form of irony in which something is intentionally represented as less than it is: “Hank Aaron was a pretty good ball player.”