"Messing with Death"
Necromancer is a common name for wizards using magic that is related to death. The most typical interpretation of this is to make them able to anime dead as skeletons, zombies, ghouls, wights and other icky creatures. However, it is not they only thing they can do. Although necromancers are typically thought as evil (likely because they mess with the dead; that's a Bad Thing. The reason for this, essentially, is that corpses are unhygienic, no matter what excuses religions offer), necromancy can also mean its opposite, messing with life; helping life is blocking death, so healing spells could be as well necromancy. At least, this is the interpretation used in Baldur's Gate.
Also, the possible offensive spells they possess are almost always about cold and death and such, for obvious reasons.