An unspecified, vaguely disreputable person.
A funny person.
A friendly unit that acts as a suspected hostile unit in a military excercise.
A jester.
A person who makes jokes.
A clause in a contract that undermines its apparent provisions.
A man.
A playing card that features a picture of a joker (that is, a jester) and that may be used as a wild card in some card games.
An uncontrolled or unpredictable element.
A character that takes the place of any other character or string that is not known or specified.
An element, often deliberately concealed, which is withheld for contingency.