The word has drifted twice and both drifts matter.
In Greek it meant divination by the dead, and that is what the classical instances are: consultation of a shade for information, at a designated place, with prescribed offerings. In medieval Latin the first element was assimilated to niger, black, and nigromancy came to mean dark magic in general, whether or not the dead were involved. A great deal of what is catalogued as necromancy in medieval sources is conjuration of spirits with no connection to the human dead at all.
That medieval practice had a specific social location. The manuals were written in Latin, they use liturgical forms, and they assume knowledge of the mass and of the exorcistic formulas of the church. Their users were largely clerics, the substantial population of men in minor orders with literacy and no benefice, and the practice is best understood as a misuse of the church's own instruments by people trained in them.
Nineteenth-century Spiritualism is a different phenomenon and should not be filed under the same heading without a note. It was public rather than clandestine, it was often domestic and led by women, its practitioners generally believed themselves to be doing something consoling rather than transgressive, and it produced an organised religious movement with its own churches. The medieval conjuror sought information under compulsion; the seance sought contact.
The atlas keeps both, and keeps them apart.