What does MLD stand for?
All of the top hits on Google refer to MLD as "mass land destruction", i.e. cards like Armageddon.

Or, to quote one of the moderators on MTGSalvation:


  MLD is mass land destruction, so generally speaking, any card that destroys multiple lands at once. This is usually indiscriminate, so Ruination is considered MLD while Sylvan Primordial is not. Back to Basics, and things that prevent you from using lands are not MLD, simply resource denial. At least, that's my take. I don't think the term has made its way into Webster's yet.


So you're right, it's not an official Magic: The Gathering term.