The False Dilemma is the fallacy of presenting an incomplete set of options (usually only two, which superficially appear to be opposites). Formally, it is the act of asserting the truth of the expression 'E1 or E2 or ... or En' in place of 'E1 or E2 or ... or Em', where m>n.
The False Dilemma is commonly employed by politicians to encourage support. For example, US President George W. Bush famously stated "You are either with us or with the terrorists".
To show the fallacy has occured, an additional option, distinct from those given, must be provided. In the above example, an additional option would be to support the policy of the French government (which opposed military intervention in Iraq, but took action against al-Qaeda members within French borders).
The fallacies of The Argument From Ignorance , The Complex Question and The Slippery Slope are special cases of The False Dilemma.
A note on the continuum
The False Dilemma is the most common logical fallacy, as in almost all cases there are more options than immediately apparent. Consider, for example, the most common question seen on a form: male or female? Even if we ignore the cases of transsexuals and transvestites, there are numerous genetic conditions (including possession of more than two sex chromosomes and non-expressiveness of Y-chromosome genes) that lead to someone being neither male nor female.