Negro is an archaic term for dark-skinned Humans of African descent. (DS9 episode: "Far Beyond the Stars")
History and specifics[]
Because of the planet Earth's long history of discrimination and injustice towards those of this heritage, this term fell into disuse as it had frequently been used as an uncomplimentary slur.
When Geordi LaForge met the hologram of Isaac Newton, Newton apologized for staring but commented that he had never met a 'Negro' before. LaForge replied that neither had he. (TNG novelization: Descent)