Argumentum Ad Hominem(Ad Hominem for short)
An ad hominem fallacy is an argument that rests on the attributes, character or motives of an opponent (usually in the form of an insult). Not all ad hominem arguments are strictly fallacious. For instance, if an attribute of your opponent is relevant to the validity or truth value of his or her argument or the matter under debate, then that can be a valid basis for whole or partial rebuttal. Also, even where personal remarks are made about an opponent, it does not follow that these amount to ad hominem if the argument put does not rest on them.
(i). "That is a stupid argument." - This comment may be fallacious in other ways, but it is not ad hominem as it does not relate to the personal attributes of the opponent.
(ii). "You are stupid." - This is ad hominem but if the intellectual capacity of the opponent is relevant to the validity of his argument, then it is not strictly fallacious. If, on the other hand, there is no argumentative relevancy in the comment, then it is fallacious.
(iii). "You are wrong, the Earth travels around the Sun. You can tell by the way that sunlight tilts. That you would think otherwise simply demonstrates that you are stupid." - This is not ad hominem because the rebuttal does not rest on the insult.
(iv). "You are wrong, the Earth travels around the Sun. You can tell by the way that sunlight tilts." - This is a substantive rebuttal, and though it might be weak or incorrect or fallacious in other ways, it is not ad hominem.