sarcasm (n.) the use of remarks that clearly meanthe opposite of what they say, made in order to hurt someone's feeling or critisize something in a humorous way
sarcastic (a.)
understatement (a.) a statement that describes something in a way that makes less important, serious, bad, etc. than it really is, or the act of making such statements
ex: To say theat her resignation was a shock would be an understatement - it caused panic.