1. A misleading, deceptive or false notion.
2. An argument that uses poor reasoning.
In common usage the term can mean any idea which is false or misleading such as “That the world is flat was at one time a popular fallacy”.
In Philosophy and Ethics we are more interested in the second use of the term; an argument that uses poor reasoning. Last week the word of the week was Syllogism (see here). Sometimes Syllogisms that look good prove to be false; these are known as deductive fallacies.
This looks like a good syllogism at first glance - but it is in fact fallacious.
“That creature” may well be a pig, but the conclusion does not follow from the premises. In fact it could be a shrew, aardvark, or even a crocodile. The problem is that statement 1 has been reversed; in this case “all pigs have snouts” is being read as “all snouted animals are pigs”. It is a common misconception that the only snouted animals are pigs, and so at first glance the syllogism works. But when you think about it more carefully you see that it is an example a deductive fallacy; individually the steps appear logical, but when placed together is actually incorrect