A limited number of options (usually two) is given, while in reality there are more options. A false dilemma is an illegitimate use of the "or" operator.
Putting issues or opinions into "black or white" terms is a common instance of this fallacy.
(i) Either you're for me or against me.
(ii) America: love it or leave it.
(iii) Either support Meech Lake or Quebec will separate.
(iv) Every person is either wholly good or wholly evil.