Pronunciation: (chal'in-jur),[key] —n. 1. a person or thing that challenges.
2. Boxing.a boxer who fights a champion for his championship title.
3. Radio.interrogator (def. 2).
4. (cap., 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.