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

Character level

From Diablo 2 Wiki

Character level, also known as Clvl or cLevel, is the uncomplicated experience level of a player character. This term becomes important in terms of choosing skills, or equipping items, where a minimum Clvl often is employed together with minimum requirements of other statistics.