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 1 1 2 min. after launch on Jan. 28, 1986, causing the death of all seven on board.

noun

contender , darer , defier , disputer