Deception (or mystification) is to intentionally distort the truth in order to mislead others. Deception is involved in propaganda and game theory (to deceive the opponents). Deception, rather than falsehood, is the essence of the lie. Thus, fiction is not true by definition, but is not taken as a lie or a deception. The reader of fiction has the duty to protect himself from deception. What one believes to be true is not a lie, though not true. Thus, freedom of speech gives the right to express statements that are in fact not true, and which may or may not have the intent to deceive.

In many cases it is difficult, even in hindsight, to distinguish deception by somebody or an organization from providing unintentionally wrong information. One of the reasons is that a person or an entire organization may be self-deceived. Deception is a reality and therefore a fascinating paradox.

There are two classes of deception: concealing the truth (dissimulate or gloss over) and exhibiting false information (simulate).