A self-fulfilling prophecy is a prediction that, in being made, actually causes itself to become true. For example, in the stock market, if it is widely believed that a crash is imminent, this may reduce confidence and actually cause such a crash. Or, if a candidate in an election openly declares he does not believe he can win, this may increase voter apathy and result in poor support for his campaign.
Self-fulfilling prophecy is sometimes seen as a manifestation of positive feedback in human society. In short, because a given prophecy was known, and was sufficiently credible, it affected people's actions and caused itself. Robert K. Merton is usually acknowledged as the maker of this phrase and using it in sociology.
Another example is Moore's law in semiconductor industry.
Examples abound in studies of cognitive dissonance theory and the related self-perception theory; People will often change their attitudes to come into line with what they profess publicly.
Other specific examples discussed in psychology include: