Understatement

Definition: A form of speech in which a lesser expression is used than what would be expected.

Understatement is a staple of humor in English-speaking cultures, especially in British humour. In Monty Python's The Meaning of Life, a suburban dinner party is invaded by Death, who wears a long black cloak and carries a scythe. He is the Grim Reaper; the party is over; the guests must all go with him. "Well," says one party guest, "that's cast rather a gloom over the evening, hasn't it?" In another scene, an Army officer has just lost his leg. When asked how he feels, he looks down at his bloody stump and responds, "Stings a bit."
- Wikipedia.

I walked out today to check on all that grows within my garden and was stopped in my tracks by this wall-to-wall, rampant display of color. When I planted this Red Creeping Thyme last June, the seven plants were about three inches in diameter and had not flowered because they were so small. Their tags modestly stated that they should be planted twelve inches apart since they could spread that much and that they would flower generously the next June.

Well, that is a clearly an understatement!