meiosis [maɪˈoʊsɪs] n.
1.) A figure of speech in which something's importance is intentionally understated or implied to be less significant or substantial than it really is. The understatement actually heightens the force of the statement (Garner's Modern American Usage 3rd Edition).
Etymology: Greek µείωσις lessening, from µειοῦν, to lessen, from µείων less.
"'No, everybody's fine at home,' I said. 'It's me. I have to have this operation.'
'Oh! I'm so sorry,' she said. She really was, too. I was right away sorry I'd said it, but it was too late.
'It isn't very serious. I have this tiny little tumor on the brain.'
'Oh, no!" She put her hand up to her mouth and all.
'Oh, I'll be all right and everything! It's right near the outside. And it's a very tiny one. They can take it out in about two minutes.'
Then I started reading this timetable I had in my pocket. Just to stop lying. Once I get started, I can go on for hours if I feel like it. No kidding. Hours" (The Catcher in the Rye, J. D. Salinger, 1951).