Literally, the ability to communicate with the dead. Necromancy is usually termed a magical or psychic ability to speak to those who are dead. It is unusual though not unheard of for someone to be able to invent or formulate an artificial method of necromancy.
Necromancers are those who possess the ability to speak with the dead, and this can vary greatly. Some may be able to bring visions of the deceased to the physical plane, and speak with them face to face. Others are able to astral project to the "other side", and communicate that way. Still others can hear echos of the dead, see ghostly apparitions others can not, sense the presence of souls or bodies, and various other similar abilities. In addition, the potency of the ability fluctuates between individuals too. Some may only be able to speak to one at a time, or multiple. The effort of communicating may put strain on the body of the necromancer, or cause them mental harm. It may even be that the act of necromancy may cause inadvertent spectral abilities, such as causing the dead person to manifest as a vengeful spirit, or forcefully inhabit the body of the necromancer.
Necromancy can also refer to, somewhat incorrectly, to the ability for one to summon the dead. In no aspect of the word "Necromancy" does the word's meaning indicate the ability to raise the deceased. It literally means to communicate with the dead.