Domain is a unit of parallelism in Multicore OCaml - it's effectively a heavyweight thread. The intention is that you tend to have as many Domains as you have cores you want to use in the computation.

Deferred relates to concurrency rather than parallelism, that you can have multiple overlapping computations. You can have concurrency without parallelism. OCaml has ways you can have parallel computations going on that don't hold the interpreter lock and I think some of the concurrent libraries can utilise this.

Multicore OCaml splits parallelism and concurrency, the former is via Domains and the latter is with Fibers. The paper I linked in my other comment on this thread touches on this a little but kc also has a short write-up of how you can use effects to write a scheduler for multicore's fibers: https://kcsrk.info/ocaml/multicore/2015/05/20/effects-multic...