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MLVL is, quite simply, a shortened version of "monster level".

Monster level is a fairly important function in the Diablo games as it pertains to item generation and drop tables. While not much is known about the monster level as it pertains to affects on items or gameplay in Diablo III, there is a rather extensive article on how it was used in Diablo II on the D2 wiki.

In addition to this, it has been revealed that the mlvl of monsters in the new Inferno difficulty mode will be from 61 to 63 or, in other terms, always higher than the maximum player level.