A throwback was a term for an individual or group who retained qualities from an earlier era that made them different or incompatible with the current one.
In 2268, Alexander apologized after describing James T. Kirk, as well as himself, as a throwback, which he claimed the Platonians referred to him as due to his lack of psychokinetic powers. Kirk didn't take any offense at the term. (TOS: "Plato's Stepchildren")
In 2365, William T. Riker referred to the seemingly simpleminded Pakleds as "curious throwbacks". (TNG: "Samaritan Snare")
In 2368, Soren self-identified as a throwback to the era when J'naii had genders, claiming to be inclined towards femininity. (TNG: "The Outcast")