understatement

noun3

1. •
   An instance of such phrasing or lack of emphasis; an incomplete statement.

   • Now that's an understatement.
2. •
   (rhetoric) 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.

   • And that mode of expression is very demurring and self-deprecating. Victorian English was all about understatement. — 2004
3. •
   An incomplete disclosure that intentionally withholds relevant information.