Uhura explains to Lincoln that mankind no longer fears words.
Negro was a term used for people of African descent throughout the English-speaking world until the mid-20th century.
During Benjamin Sisko's vision of 1950s America, he and other people of color were often referred to as Negroes. Herb once joked about the dangers of a Negro with a typewriter. Jimmy suggested that people of color would never get into space except as shoe shiners, because "as far as they're concerned, we'll always be niggers." (DS9: "Far Beyond the Stars")
The Excalbian recreation of Abraham Lincoln referred to Uhura as "a charming Negress" before realizing that his words were poorly chosen. He corrected himself by stating that in his time, "some used that term as a description of property." Uhura, however, asked why she should object to that term, explaining that Human philosophy of the 23rd century taught mankind not to fear words and to be delighted with who they were. (TOS: "The Savage Curtain")