The tendency to seek pleasure and avoid pain. In Freud's theory, this principle rules the Id, but is at least partly repressed by the 'reality principle'. The origin of this expression can be traced to G Th. Fechner (1801-87), who used the German equivalent Lustprinzip in the defined sense in an article published in 1848. The theory that all action is determined by the prospect of pleasure is called psychological hedonism.
The Penguin Dictionary of Philosophy
ed. Thomas Mautner
Critique of Brave New World
The Cyrenaics and the Origin of Hedonism