Fallacy: Noun

Definition:  a wrong belief : a false or mistaken idea.

Source: http://www.merriam-webster.com/dictionary/fallacy

I heard this word while I was in class in professor Davis class. He was teaching the  class different fallacies there is in public speaking.

Now that I understand what a fallacy is I know that while giving a speech you should avoid fallacies which means to avoid giving your audience a mistaken idea.