An informal fallacy is an logical argument whose premises fail to support their conclusion. Informal fallacies often come from errors in reasoning.
In contrast to a formal fallacy, the error has to do with issues of inference. Because language is used to state the propositions, natural language allows one to say more than formal logic can represent. All informal fallacies of deductive reasoning contain a disconnect between the premises and the conclusion. This makes the argument invalid. The disconnect happens because there is a hidden co-premise. If this co-premise were stated, it would make the argument valid.