A boxer was an athlete who competed in the sport of boxing.
Chakotay was known as a proficient boxer at the light heavyweight level with twenty-three wins and one loss. He was known as "The Maquis Mauler" and "The Tattooed Terror" when he boxed. His personal trainer was Starfleet Academy groundskeeper Boothby. (VOY: "The Fight", "Tsunkatse")
In 2367, Noonian Soong, upon being asked on Terlina III by Data why he created him, asked rhetorically, "Why does a painter paint? Why does a boxer box?" (TNG: "Brothers")
In a line cut from "A Piece of the Action", Kalo, upon seeing Spock's ears, asked him if he was a boxer.