a. A person who can beat well over 99% of other chess players. This excludes non-chess players.
b. A person who has a chess rating of 2200 or higher, awarded by the United States Chess Federation, or a chess rating of 2300 or higher awarded by the World Chess Federation. Or awarded by his or her country by demonstrating outstanding tournament performances over a period of time.
NOTE: Having a 2200+ chess rating in any type of chess software does not give one a master status. S/he must earn the master title by competing in real life, over-the-board tournaments.
A chess master is also someone who is very knowledgeable of the game of chess and has true talent.
So he walked into a tournament filled with 5,000 players and he won 1st place. Wanna bet he's a chess master?