I first heard the term mega drop-down from usability expert, Jakob Nielsen. Not sure it's a technical term, but it refers to hover-over, drop-down navigation extraordinaire that has categories, columns, links and more. It's super helpful, and it's a key component of Yahoo!'s new interface.
The new Yahoo! homepage sports a "favorites" bar along the left hand side. Icon/links for key areas of interest like sports, finance, facebook, eBay and a dozen or so other topics are included. Here's the mega part: when you hover over any of the icons, a mega drop-down pops out, with a few key piece of info -- stuff you're most likely to want to see (or at least that's what the designers are banking on). For example, a quick hover gives you the state of the Dow, Nasdaq and S&P. Place your mouse over eBay, and you'll see the most-watched items, and a search box. Facebook gives you direct access to your login. True to its original intent, Yahoo! has once again made itself a reason to be your portal.
This is also a brilliant move for advertising, as each of the mega drop-downs has room for an ad... which of course, is targeted to the area of interest. Nice job of combining user benefits with money-making options for the site.
— Andrea Zuegel