competitor - a person (or group) that is in competition with another person (or group), trying to succeed by making the other fail
Contestant - a competitor in a structured game or contest. Businesses that sell the same product are competitors but not contestants
contester - a person who contests (opposes, protests, disputes) the outcome of a decision
contender - a competitor or contestant who has a good chance of winning a specific competition (a contender for the world championship)