I have found that towards the end of his career, Foucault offered particularly clear and useful definitions of a number of concepts. I should add the caveat perhaps, that Foucault often redefined and refined concepts at different points in his writings. I like this passage which defines the differences in the way philosophical and spiritual systems approach the truth:
We will call, if you like, “philosophy” the form of thought that asks, not of course what is true and what is false, but what determines that there is and can be truth and falsehood and whether or not we can separate the true and the false. We will call “philosophy” the form of thought that asks what it is that enables the subject to have access to the truth and which attempts to determine the conditions and limits of the subject’s access to the truth. If we call this “philosophy,” then I think we could call “spirituality” the search, practice, and experience through which the subject carries out the necessary transformations on himself in order to have access to the truth. We will call “spirituality” then the set of these researches, practices, and experiences, which may be purifications, ascetic exercises, renunciations, conversions of looking, modifications of existence, etc., which are, not for knowledge but for the subject, for the subject’s very being, the price to be paid for access to the truth.
Michel Foucault, The Hermeneutics of the Subject: Lectures at the Collège de France, 1981–1982, ed. Frédéric Gros, trans. Graham Burchell, English series ed. Arnold I. Davidson (New York and Basingstoke: Palgrave Macmillan, 2005), p.15