Definition:
generally refers to changing the subject mid-debate, so that we start arguing about a tangential topic rather than the real or original issue.

Red Herring

Example: Ex: We start debating the evidence supporting evolutionary theory, but you bring up the fact that believing this theory is depressing.
Ex: We start debating the evidence supporting global warming, but you bring up the fact that believing this theory is depressing...or that Al Gore has a big house and flies on jets a lot.