non sequitur

A non sequitur is any argument that does not follow from the previous statements. Usually what happened is that the writer leaped from A to B and then jumped to D, leaving out step C of an argument she thought through in her head, but did not put down on paper. The phrase is applicable in general to any type of logical fallacy, but logicians use the term particularly in reference to syllogistic errors such as the undistributed middle term, non causa pro causa, and ignorantio elenchi. A common example would be an argument along these lines: "Giving up our nuclear arsenal in the 1980's weakened the United States' military. Giving up nuclear weaponry also weakened China in the 1990s. For this reason, it is wrong to try to outlaw pistols and rifles in the United States today." There's obviously a step or two missing here.