Logical fallacy/Ignoratio elenchi

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

For example, the prosecuter 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 guilty.

To prove an argument is ignoratio elenchi one needs to show that the conclusion the arguer proves is not the one he set out to prove.

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[Stephen's Guide: Irrelevant Conclusion]