The tendency to do the opposite of what someone wants you to do because you think they are trying to constrain your freedom of choice.
Reactance can occur when someone pressures you to accept a certain viewpoint. It can cause you to actually strengthen your stance. For example, do teens drink in excess in an environment of prohibition when they wouldn't do so in a more permissive culture.
People who use "reverse psychology" are playing on an awareness of reactance, attempting to influence someone to choose the opposite of what they request. This is a frequent method used in fraudulent or unethical sales pitches, manipulating a consumer into choosing an option they would not necessarily buy.