Red herring is an argument, given in reply, that does not address the original issue. Critically, a red herring is a deliberate attempt to change the subject or divert the argument.
Examples of Red herrings
If, in a debate on the existence of god, a theist states:
"God must be real, because if god were not real, I would have no reason to go on living."
Here, the theist makes a claim that provides no further evidence for the existence of a god, and in fact, makes a claim that leads to an entirely different topic of conversation.