https://en.wikipedia.org/wiki/Understatement

Understatement is an expression of lesser strength than what the speaker or writer actually means or than what is normally expected. It is the opposite of embellishment or exaggeration, and is used for emphasis, irony, hedging, or humor.

Understatement also merges the comic with the ironic, as in Mark Twain's comment, "The report of my death was an exaggeration."