Understatement is a figure of speech that deliberately presents something as less important, less severe, or less impressive than it really is. Poets use it to create irony, restraint, wit, or emotional control, often by saying far less than the situation calls for.
It is related to litotes, but understatement is broader: it may rely on gentle wording, while litotes specifically uses a negative to express the positive, such as saying "not bad" to mean "very good."
Example
I missed the train, lost my wallet, and tripped in the rain-
just not the smoothest morning I've had.
[n] a statement that is restrained in ironic contrast to what might have been said