Here is a long ariticle on fallacies. It's long because it lists most of the common fallacies -- of which there are a lot. They give four definitions of a fallacy:
Whichever definition of 'fallacy' one adopts, it means that the reasoning is flawed and needs to be revised.The term "fallacy" is not a precise term. One reason is that it is ambiguous. It can refer either to (a) a kind of error in an argument, (b) a kind of error in reasoning (including arguments, definitions, explanations, etc.), © a false belief, or (d) the cause of any of the previous errors including what are normally referred to as "rhetorical techniques". Philosophers who are researchers in fallacy theory prefer to emphasize meaning (a), but their lead is often not followed in textbooks and public discussion.
I highly recommend reading through the list of fallacies. Most of us here (myself included) rely more on fallacies than on sound argument.