https://web.mit.edu/allanmc/www/foucault1.pdf

First there are the utopias. Utopias are sites with no real place. They are sites that have a general relation of direct or inverted analogy with the real space of Society. They present society itself in a perfected form, or else society turned upside down, but in any case these utopias are fundamentally unreal spaces.

There are also, probably in every culture, in every civilization, realplaces—places that do exist and that are formed in the very founding of society—which are something like counter-sites, a kind of effectively enacted utopia inwhich the real sites, all the other real sites that can be found within the culture,are simultaneously represented, contested, and inverted. Places of this kind are outside of all places, even though it may be possible to indicate their location inreality. Because these places are absolutely different from all the sites that theyreflect and speak about, I shall call them, by way of contrast to utopias,heterotopias. I believe that between utopias and these quite other sites, theseheterotopias, there might be a sort of mixed, joint experience, which would be themirror. The mirror is, after all, a utopia, since it is a placeless place. In the mirror,I see myself there where I am not, in an unreal, virtual space that opens up behindthe surface; I am over there, there where I am not, a sort of shadow that gives myown visibility to myself, that enables me to see myself there where I am absent:such is the utopia of the mirror. But it is also a heterotopia in so far as the mirrordoes exist in reality, where it exerts a sort of counteraction on the position that Ioccupy. From the standpoint of the mirror I discover my absence from the placewhere I am since I see myself over there. Starting from this gaze that is, as itwere, directed toward me, from the ground of this virtual space that is on theother side of the glass, I come back toward myself; I begin again to direct myeyes toward myself and to reconstitute myself there where I am. The mirrorfunctions as a heterotopia in this respect: it makes this place that I occupy at themoment when I look at myself in the glass at once absolutely real, connectedwith all the space that surrounds it, and absolutely unreal, since in order to beperceived it has to pass through this virtual point which is over there.

As for the heterotopias as such, how can they be described? Whatmeaning do they have? We might imagine a sort of systematic description—I donot say a science because the term is too galvanized now—that would, in a givensociety, take as its object the study, analysis, description, and ‘reading’ (as somelike to say nowadays) of these different spaces, of these other places. As a sort ofsimultaneously mythic and real contestation of the space in which we live, thisdescription could be called heterotopology.

1. Its first principle is that there is probably not a single culture in the world that fails to constitute heterotopias.
2. The second principle of this description of heterotopias is that a society, as its history unfolds, can make an existing heterotopia function in a very different fashion; for each heterotopia has a precise and determined function within a society and the same heterotopia can, according to the synchrony of the culture in which it occurs, have one function or another.
3. Third principle. The heterotopia is capable of juxtaposing in a single real place several spaces, several sites that are in themselves incompatible.
4. Fourth principle. Heterotopias are most often linked to slices in time—which is to say that they open onto what might be termed, for the sake of symmetry, heterochronies.
5. Fifth principle. Heterotopias always presuppose a system of opening and closing that both isolates them and makes them penetrable.
6. Sixth principle. The last trait of heterotopias is that they have a function in
relation to all the space that remains.

The idea of the Heterotopia is pretty powerful and useful, but by the beard of Zeus was this a convoluted read. Perhaps it's the translation, perhaps it's because it's transcribed - either way, skip the article, read the fine Wikipedia page and avoid this labyrinthe. ( )