An understatement is a figure of speech used in writing and speaking that diminishes (understates) the seriousness or importance of an event or situation. It is the opposite of hyperbole/overstatement.
An understatement creates the effect of irony, since the response is contrary to what is expected in a severe situation. Understatement can also be accompanied by an ironical tone. The device can be used to deliberately decrease the intensity of an event or situation.
Writers should not underestimate the effect of understatement.
Holden Caulfield in Salinger’s Catcher in the Rye: “I have to have this operation. It isn’t very serious. I have this tiny little tumor on the brain.”
See more examples for understatement here at literarydevices.net.
(Source: Understatement, Literary Devices, Definition and Examples of Literary Terms, literarydevices.net.)