Foucault defined his use of the term dispositif (apparatus) in 1977:
What I’m trying to pick out with this term is, firstly, a thoroughly heterogenous ensemble consisting of discourses, institutions, architectural forms, regulatory decisions, laws, administrative measures, scientific statements, philosophical, moral and philanthropic propositions–in short, the said as much as the unsaid. Such are the elements of the apparatus. The apparatus itself is the system of relations that can be established between these elements.
Secondly, what I am trying to identify in this apparatus is precisely the nature of the connection that can exist between these heterogenous elements. Thus, a particular discourse can figure at one time as the programme of an institution, and at another it can function as a means of justifying or masking a practice which itself remains silent, or as a secondary re-interpretation of this practice, opening out for it a new field of rationality.
In short, between these elements, whether discursive or non-discursive, there is a sort of interplay of shifts of position and modifications of function which can also vary very widely.
Thirdly, I understand by the term “apparatus” a sort of–shall we say–formation which has as its major function at a given historical moment that of responding to an urgent need. The apparatus thus has a dominant strategic function. This may have been, for example, the assimilation of a floating population found to be burdensome for an essentially mercantilist economy: there was a strategic imperative acting here as the matrix for an apparatus which gradually undertook the control or subjection of madness, sexual illness and neurosis.
“The Confession of the Flesh” (1977) interview. In Power/Knowledge Selected Interviews and Other Writings (ed Colin Gordon), 1980: pp. 194-228. This interview was conducted by a round-table of historians.
Update (2/13/14): This term is experiencing some interesting deployments in the resilience literature. See Bruce Braun here, who draws from the 1977 interview quoted above. Clare O’Farrell adds earlier examples, including Psychiatric Power 7 and 28th Nov. 1973, “From torture to cellblock” in 1975 (in Foucault Live). We might also note its multiple use in Discipline and Punish.