Ignoratio elenchi

Ignoratio elenchi (which can be roughly translated by ignoring the issue; also known as "irrelevant conclusion") is the logical fallacy of presenting an argument that may of itself be valid, but which proves a different proposition than the one it is supposed to prove.

For example, the prosecutor in a criminal court might argue how bad the crime is that has been committed, but if this argument is taken as an argument to convict the suspect, it is a case of ignoratio elenchi--it does not prove what it should prove, namely that the suspect is the one who committed it.

External link: Stephen's Guide: Irrelevant Conclusion (http://www.intrepidsoftware.com/fallacy/irrelev.htm)