Is a fallacy considered an argument that makes a mistake in deriving a conclusion?

Yes.

http://www.google.com/search?hl=en&noj=1&q=fallacy&tbs=dfn:1&tbo=u&sa=X&ei=0KaNUOHqKcTAyAHKwYHQDw&ved=0CB4QkQ4&biw=711&bih=453