Actually HNM is Honorable Notorious Monster. A HNM is any NM that gives you a title upon killing it. If it were Hard Notorious Monster then Hoo would be a HNM to low levels.
Well... most people consider HNM to stand for Hyper Notorious Monster. Poof spent a fortnight screaming in Jeuno that the term is Japanese for Honorouble Notorious Monster which kinda makes sense since a majority of the HNMs do give titles, but not all of the mobs that give titles are HNM eg. Stray Mary. There is no true way of defining a HNM since there are a number of factors and not all of them are covered to say that it is truely a HNM. Any combination of these factors can suggest a particular NM is HNM:
1) It requires more than 1 L70 to kill it (not all HNMs fall under this category, some can be solod by a L75 and some can be killed with a party of L65+.) 2) It respawns 21+ hours from last TOD (a true statement for all HNMs, although some NMs can fall under this category also.) 3) It drops a rare item that is worth several million gil (a true statement for most HNMs, there are some that are rare/ex drop only that if they had a monetary value would be worth several million ie. Voluptuous Vivian.) 4) It uses a 2hr ability (not all HNMs use 2hr abilities.) 5) Killing it gives you a title (most HNMs will give you a title, however some NMs also fall into this category eg. Stray Mary.)
Ok obviously you guys missed the article where SE stated that HNM stands for "High-Level Notorious Monster". I still call it "Huge Notorious Monster" though because that's how I learned it. Just thought you should know.