What does "liquidated damages" mean in the context of a NY breach of contract ?

As noted in "How to Challenge the Validity of a Liquidated Damages Clause Under New York Law," the better question is as follows: when is a clause in a contract that provides for a sum certain as an amount of damages considered enforceable (i.e., liquidated damages) as opposed to an unenforceable (or in legal terms, "unconcscionable") penalty?