Red Herring

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A Red Herring is a proposition within an argument that seems to apply to the topic at hand, but is actually unrelated. Arguments which rely on a red herring may be perfectly sound, but can reach only an Irrelevant Conclusion. They are a subclass of (and almost synonymous with) Ignoratio Elenchi, or fallacies of relevance.

Bruce Thompson:

The speaker introduces a new subject into the discussion that has a superficial similarity to the topic under discussion. The new subject is so emotionally charged that people cannot resist arguing about it, even though it is off the original subject. Raising the new topic does not really serve the goal of bringing the original subject to a conclusion (i.e. of getting the audience to accept the speaker's position). Rather, it distracts attention away from the original subject, preventing either side from supporting its conclusion.

Fallacy Files:

This fallacy is often known by the Latin name Ignoratio Elenchi, which translates as "ignorance of refutation". The ignorance involved is either ignorance of the conclusion to be refuted—even deliberately ignoring it—or ignorance of what constitutes a refutation, so that the attempt misses the mark.
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(The name of this fallacy comes from the sport of fox hunting in which a dried, smoked herring, which is red in color, is dragged across the trail of the fox to throw the hounds off the scent. Thus, a "red herring" argument is one which distracts the audience from the issue in question through the introduction of some irrelevancy.)