Terminological questions are important in philosophy. As a philosopher for whom I have the greatest respect once said, terminology is the poetic moment of thought. This is not to say that philosophers must always necessarily define their technical terms. Plato never defined idea, his most important term. Others, like Spinoza and Leibniz, preferred instead to define their terminology more geometrico.
The hypothesis that I wish to propose is that the word dispositif, or “apparatus” in English, is a decisive technical term in the strategy of Foucault’s thought. He uses it quite often, especially from the mid 1970s, when he begins to concern himself with what he
calls “governmentality” or the “government of men.”
Agamben, Giorgio. What is an Apparatus? And Other Essays. Translated by David Kishik and Stefan Pedatella. Stanford, CA: Stanford University Press, 2009.