challenger

Short definition from Wiktionary

Noun

1. 1
   One who challenges; especially, one who plays against the current champion of a game or contest in hopes of winning and becoming the new champion.
2. 2
   a steam locomotive of the 4-6-6-4 wheel arrangement.

Proper noun

1. 1
   A space shuttle, named after HMS Challenger (1858), destroyed on January 28, 1986 with loss of its seven-member crew.