As you are aware, the nature of a question determines what is possible as an answer. Thus to ask what “is” a domain is giving it an ontological status of pre-existing as itself; whether as a tangible object or a specific notion. –– Domains Co-arise With Distinctions page 
YOUTUBE 18GHIbYj1_M Pille Bunnell's What Is a Domain
However, we can still ask a question that provides a satisfying and even rigorous answer that helps us understand our living. Namely we can once more convert the question from “being” (ie. what IS it) to “doing”, namely what is it that we are doing such that this “thing” that we can refer to as a “domain” arises as part of our living.
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Servers are allocated IP addresses which then must be associated with a domain name which one leases for a few years at a time. A domain name registrar is a company that manages the reservation of Internet domain names. wikipedia