Incumbent: An incumbent is a current officeholder. In this case, Bill Clinton was the incumbent because he was already serving as president during the election.
Nominee: A nominee is a person who has been proposed or formally entered as a candidate for an office or award. In this context, both Bob Dole and Bill Clinton were nominees for their respective parties.
Reform Party: The Reform Party is a moderate, centrist political party in the United States that was founded in 1995 by Ross Perot.