I just noticed that this article by Latour: 'Paris, invisible city: The plasma' -- is now available from 'City, Culture and Society.' Or, rather, it is available in unfinalised form. The issue in question was seemingly meant to be published in March 2012 but, rather typically of academic publishing, it is months behind schedule. However, the papers are available in 'corrected draft' form and Latour's contribution makes for interesting reading.
He revisits (presumably at the editors' request) an electronic photo essay from some fifteen years ago: 'Paris: Ville Invisible.' This essay, co-produced with Emilie Hermant, was originally published in French in 1998. A later English translation, available on Latour's website, was never formally published and, consequently, has largely been ignored. This is a shame as it is some of Latour's best work and, in my opinion, easily the best single general introduction to Actor-Network Theory; far better in fact than 'Reassembling the Social,' which Latour wrote for that very purpose.
It is also the work that first introduced the concept of 'plasma,' which Reassembling develops more explicitly but without making especially clear. Graham Harman picked up on the concept in his 'Prince of Networks,' and highlighted its importance but, unfortunately, made a bit of a mess of articulating it (in my humble opinion).
Harman claimed that plasma is something that is totally unformatted, outside all relations, metaphysically, when it is fairly clear from the original Paris, from Reassembling and from this latest essay that plasma denotes that which is outside of relations with respect to a particular network. Typically, this means when something is outside the epistemic networks that socialise things and make them possible objects of politics. It is, therefore, more of a sociological concept than a metaphysical one; it "is what makes it possible to measure the extent of our ignorance concerning Paris" -- or the world in general. It isn't something that is generalisable to all things and all relations since it pertains primarily to epistemic or social relations.
Indeed, "ignorance" is the key word. Plasma is that which is unformatted (or formalised) and therefore cannot circulate in (our) networks -- this is what Latour claims in Reassembling. This doesn't mean that plasma is, metaphysically speaking, outside all relations. Plasma is a concept deployed to prevent premature formalism; the assumption that we already know how things hold together. It is precisely because of our profound ignorance with respect to how parts relate to wholes and how things go about their existence (and, indeed, how things affect our existence) that we mustn't prematurely shoehorn them into prescribed roles, lest we foreclose our ability to come to understand them and, in Latour's terms, to engage in composition -- i.e. politics.
It is the megalomania of the panopticon that presumes that we can sensibly speak of things that have not yet been enrolled in our networks. This is Latour's Kantian moment, however it differs from Kant as noumena are not forever, irrevocably 'out there' -- they can be enrolled, formalised and brought 'in here.'
Plasma may or may not be differentiated within itself but it is a mystery to us for as long as it remains formless, as long as it does not circulate within our networks. However, it is not so far flung and mysterious that it is forever beyond our grasp. And the first step in grasping it, it seems, is to admit our ignorance and avoid making unjustified assumptions as to what it's all about. Hence plasma.
When talking about plasma Latour clearly has his sociological hat on, not his philosophical one. Of course, there's nothing stopping anyone from extracting philosophy from a sociology and Harman does this wonderfully with Latour's work for the most part but the project falls apart somewhat when it gets to plasma. In fact it ends up with Harman completely misunderstanding Latour.
The above may not make a lot of sense if you've not read the essay (and perhaps still won't even if you have!). In truth, Latour still avoids really making the thing clear but it certainly adds a degree of clarification to his previous passing mentions of the subject. In any case, the essay is worth a read and Paris: Ville Invisible (in either language) is highly recommended reading/viewing if that has passed you by up until now.