Interestingly enough, the large and heavy book by Sangiori and Walker does not list “concurrency” in its index or table of contents, and looking into Milner’s book yields just section 4.3, which does not feature a formal definition of the term but merely introduces composition (i.e. concurrent processes) and summation (i.e. alternatives). The distinguishing feature of the composition syntax is to allow reactions to take place; in fact it is the only syntax allowing reactions. My layman’s interpretation of this fact is that concurrency is intimately linked to reactions, supporting my earlier argument.