U

challenger

noun

a person or thing that challenges.
Boxing. a boxer who fights a champion for his championship title.
Radio. interrogator(def 2).
(initial capital letter, italics) U.S. Aerospace. the second space shuttle to orbit and return to earth: exploded 1½ min. after launch on Jan. 28, 1986, causing the death of all seven on board.

late 13c., “a claimant;” mid-14c., “one who makes false charges;” mid-15c., “one who disputes something, objector,” from Anglo-French chalengeour (Old French chalongeor “slanderer, petitioner, plaintiff”), agent noun from challenge (v.). Specific sense of “one who calls out another in a contest” is from 1510s.