Ancestor worship is a phrase with a source. Herbert Spencer used it in 1876 to name what he took to be the original religion, out of which everything else had grown, and although the theory collapsed the label stayed. Missionaries and colonial officers took it up as plain description, carrying an implication the people described almost universally denied: that they were treating their dead relatives as gods.
The denial is not a modern apologetic; it is built into the traditions. The Akan distinguish the nsamanfo from the abosom, the deities; the Igbo place the ndichie below the alusi and both well below Chukwu; the Zulu do not address the amadlozi in the terms used for uNkulunkulu. In each case what is owed to the dead is stated in the vocabulary of kinship obligation, not of divinity. John Mbiti made this the centre of his argument in 1969, insisting that the English word worship was the wrong instrument, and proposed calling the ancestors the living dead. His term has itself been criticised for producing exactly the pan-African uniformity the traditions do not display.
Two corrections belong here. The first is that the doctrine is not African. Roman households kept the di manes and carried wax masks of their forebears in funeral processions; Chinese families keep the ancestral tablet; Japanese Buddhism organises much of its practice around the family dead; Polynesian traditions do the same. A comparative account that files ancestor veneration under Africa describes the history of anthropology rather than the distribution of the belief.
The second is practical. Whether feeding an ancestor is worship or remembrance determines whether a Christian or Muslim convert may continue, and millions have settled that for themselves in ways their clergy have not always endorsed.