Understatement
Etymology
A plain English compound: under (Old English under, "beneath, below") + statement. Unlike most rhetorical terms, it never passed through Greek or Latin — it is a native-built name for saying less than the case warrants. Classical rhetoric called the same figure meiosis, "a lessening."
noun · /UN-der-state-ment/
Describing something as less significant than it really is, for effect.
What it names
The composure of calling a catastrophe "a bit of a setback" — restraint that can read as wit, stoicism, or barely-hidden alarm.
Example
Surveying the flooded kitchen, he managed a masterpiece of understatement: "We may have a small problem."
Related idea
Litotes is a narrower species that understates specifically by negation ("not great"); all litotes is understatement, but plain understatement need not negate anything.