In Speculative Realism, Correlationism is defined as the term linking Thinking and Being.
Is the term original to this movement or picked up from post-Kantian thinking?
"Correlationalism," as a term and a critique, originates in Meillassoux's After Finitude. It is a neologism he creates. It is essentially levelled at a great deal philosophy since Berkeley and Hume, including Kant and post-Kantian philosophy. Thus, in a recent lecture where he attempts to clarify the meaning of correlationalism, Meillassoux says "I call the 'era of Correlation'...the anti-materialist, post-Berkeleyan era that shuts us up inside correlation."
Nevertheless, Meillassoux's project---and perhaps Speculative Realism more broadly---is a return to certain pre-Kantian theses, albeit with a different type of justification. Thus, in After Finitude, Meillassoux writes:
Is this to say that we must once again become pre-critical philosophers, or that we must go back to dogmatism? The whole problem is that such a return strikes us as strictly impossible—we cannot go back to being metaphysicians, just as we cannot go back to being dogmatists. Yet it seemed that we were defending a Cartesian (and hence dogmatic) thesis – viz., the distinction between primary and secondary qualities—against its critical disqualification. However, this defence—and here lies the rub—can no longer be sustained by Cartesian argumentation.
In a certain sense, then, some of the critique of correlationalism is already existent prior to Meillassoux's coinage. Thus, for example, Heidegger anticipates at least some of the aspects of the anti-correlationalist critique in §43 of Being and Time:
In so far as Reality has the character of something independent and "in itself," the question of the meaning of “Reality” becomes linked with that of whether the Real can be independent ‘of consciousness’ or whether there can be a transcendence of consciousness into the ‘sphere’ of the Real. The possibility of an adequate ontological analysis of Reality depends upon how far that of which the Real is to be thus independent—how far that which is to be transcended—has itself been clarified with regard to its Being. Only thus can even the kind of Being which belongs to transcendence be ontologically grasped. And finally we must make sure what kind of primary access we have to the Real, by deciding the question of whether knowing can take over this function at all.
These investigations, which take precedence over any possible ontological question about Reality, have been carried out in the foregoing existential analytic.
As we have noted, Being (not entities) is dependent upon the understanding of Being; that is to say, Reality (not the Real) is dependent upon care. By this dependency our further analytic of Dasein is held secure in the face of an uncritical Interpretation which nevertheless keeps urging itself upon us—an Interpretation in which the idea of Reality is taken as the clue to Dasein. Only if we take our orientation from existentiality as Interpreted in an ontologically positive manner, can we have any guarantee that in the factical course of the analysis of ‘consciousness’ or of ‘life’, some sense of “Reality” does not get made basic, even if it is one which has not been further differentiated.
Meillassoux's unique contribution, then, perhaps is less in the term correlationalism as in bringing out a term which unites a host of critiques against a wide range of philosophers.