Overdetermination

Overdetermination, the idea that a single observed effect is determined by multiple causes at once (any one of which alone might be enough to account for the effect), was originally a key concept of Sigmund Freud's psychoanalysis.

For Freud and Psychoanalysis
Freud wrote in The Interpretation of Dreams that many features of dreams were usually "overdetermined," in that they were caused by multiple factors in the life of the dreamer, from the "residue of the day" (superficial memories of recent life) to deeply repressed traumas and unconscious wishes, these being "potent thoughts". Freud favored interpretations which accounted for such features not only once, but many times, in the context of various levels and complexes of the dreamer's psyche.

The concept was later borrowed for a variety of other realms of thought.

For Richards and literature
The New Critic I.A. Richards used the idea of overdetermination in order to explain the importance of ambiguity: in rhetoric, the philosophy of language, and literary criticism.

For Althusser and Structuralist Marxism
The Marxist philosopher Louis Althusser imported the concept into Marxist politics in an influential essay, "Contradiction and Overdetermination." Drawing, in an unusual combination, from both Freud and Mao Zedong, Althusser used the idea of overdetermination as a way of thinking about the multiple, often opposed, forces active at once in any political situation, without falling into an over-simple idea of these forces being simply "contradictory." Brewster, in Althusser et al's Reading Capital defines overdetermination as such:

"the representation of dream thoughts in images privileged by their condensation of a number of thoughts in a single image (condensation), or by the transference of psychic energy from a particularly potent thought to apparently trivial things ... [For Althusser] overdetermination of a contradiction is the reflection in it of its conditions of existence within the complex whole."

Thus, for Althusser's reiterating of Marxist thought, overdetermination is what [concealed/unaccepted] "determinant contradictions", or capital-economic incongruities(i.e., abstract labour resulting in "isolation" -- the class struggle), which are analogous to Freud's "potent thoughts", apply to instances  that are more really, slight, understandable. An instance of a popular riot calling for revolution could exemplify this. The event has to it, in capitalist culture, an over-application(determination) of agitation. The determinant contradictions(the reasons for popular revolt) are not addressed and so their great mass is "displaced" onto the singular event.

For Baudrillard and Theoretical Sociology towards a Theory of Hyperreality
Another conception of overdetermination is present in the later writings of Jean Baudrillard. His derives from Althusser's conception, but breaks with it just as Baudrillard breaks with structural Marxism in general.

In mathematics
A system of linear equations is considered overdetermined if there are more equations than unknowns.