Deception or falsehood is an act or statement that misleads, hides the truth, or promotes a belief, concept, or idea that is not true. It is often done for personal gain or advantage. Deception can involve dissimulation, propaganda and sleight of hand as well as distraction, camouflage or concealment. There is also self-deception, as in bad faith. It can also be called, with varying subjective implications, beguilement, deceit, bluff, mystification, ruse, or subterfuge.

Deception is a major relational transgression that often leads to feelings of betrayal and distrust between relational partners. Deception violates relational rules and is considered to be a negative violation of expectations. Most people expect friends, relational partners, and even strangers to be truthful most of the time. If people expected most conversations to be untruthful, talking and communicating with others would require distraction and misdirection to acquire reliable information. A significant amount of deception occurs between some romantic and relational partners.

Deceit and dishonesty can also form grounds for civil litigation in tort, or contract law (where it is known as misrepresentation or fraudulent misrepresentation if deliberate), or give rise to criminal prosecution for fraud. It also forms a vital part of psychological warfare in denial and deception.