understatement

A figure of speech whereby something is made to seem smaller or less important than it actually is, either through phrasing or lack of emphasis, often for ironic effect.

A master of understatement

And that mode of expression is very demurring and self-deprecating. Victorian English was all about understatement.

An instance of such phrasing or lack of emphasis; an incomplete statement.

Now that's an understatement.

An incomplete disclosure that intentionally withholds relevant information.