According to the psychoanalyst Sigmund Freud, the drive or concept which motivates people to seek out immediate pleasure or gratification and avoid pain.
Example: Most smokers are aware of the health risks associated with cigarettes, but despite this they choose the momentary gratification of smoking in order to avoid the discomfort of withdrawal.
Grinnell, R. (2009). Pleasure Principle. Psych Central. Retrieved on April 18, 2015, from http://psychcentral.com/encyclopedia/2009/pleasure-principle/