The Diablo 2 Wiki
The Diablo 2 Wiki archive is available for anyone looking for information on Diablo 2. All information here is pre-D2R but contains a lot of useful information that is still relevant. Updated sections for new D2R features can be found on the PureDiablo Diablo 2 section


From Diablo 2 Wiki

mlvl = monster level. A monster's level is the same as the area level in which it is found. Champion monsters (blue text) are area level +2 and unique monsters (gold text) and minions are area level +3.