Wikipedia: Cardinal

In mathematics, cardinal is short for cardinal number.
In Catholicism, a cardinal is a prince of the Church; cardinals are appointed by the Pope and get to vote for the next Pope if they are under the age of eighty at the time of the election.
In biology, a cardinal is the songbird Cardinalis cardinalis. They are red, similar to the dress of cardinals in the catholic church, hence the name.
In sports, the Cardinals are a baseball team in St. Louis (see Saint Louis Cardinals), an American football team in Arizona (see Arizona Cardinals), and a college team from Stanford, California.