Necromancy (Greek νεκρομαντία, nekromantía) is a form of divination in which the practitioner seeks to summon the dead, who materialise as "operative spirits" or "spirits of divination", for multiple reasons, from spiritual protection to wisdom. The word necromancy derives from the Greek νεκρός (nekrós), "dead", and μαντεία (manteía), "divination".
However, since the Renaissance on Roundworld, necromancy has come to be associated more broadly with black magic and demon-summoning in general, sometimes losing its earlier, more specialized meaning. By popular etymology, nekromantia became nigromancy "black arts", and Johannes Hartlieb (1456) lists demonology in general under the heading.
While Eric Thurslow might have thrown himself enthusiastically and in a dangerously haphazard self-taught way into his deceased uncle's collection of grimoires dealing with necromancy and demonology, had he been accepted for formal training at Unseen University, he would have realised that necromancy now belongs to the dark age of dribbly candles and the sort of un-necessary paraphernalia that gives wizarding such a bad name. No, we are now at the dawn of a bright new age of technomancy, and in the modern world of wizardry, it is now the province of the Department of Post-Mortem Communications to perform rites that have nothing at all to do with the outmoded and dangerous practice of necromancy, oh no. Ignore what it says on the door, will you, we just haven't got round to having it repainted yet. And ignore whatever Professor Flead has to say about it, as he's dead, that's why!
Granny Weatherwax has particularly extreme views on speaking with the dead: she won't even read books in case the people who wrote them are dead, in which case she's being informed by a dead person, which is practic'ly necromancy in itself.