The term domain refers to a subsection of the Internet. As noted in RFC 920:
Domains are administrative entities. The purpose and expected use of domains is to divide the name management required of a central administration and assign it to sub-administrations. There are no geographical, topological, or technological constraints on a domain. The hosts in a domain need not have common hardware or software, nor even common protocols. Most of the requirements and limitations on domains are designed to ensure responsible administration.
The primary domains of the Internet are .com, .net, .mil, and .org, which refer to commercial, network, military, and organization, respectively.
A domain is a high-level grouping of organizations, buildings, individuals, systems, devices or other actors with similar objectives and relying on — or participating in — similar types of applications. Communications among actors in the same domain may have similar characteristics and requirements. Domains may contain sub-domains. The transmission and distribution domains have much overlapping functionality and often share networks and are therefore represented as overlapping domains.
A domain refers to
[l]ogical groupings of disciplines that form the main building blocks within the architectural framework; a sphere of activity, interest, or function.
[h]igh-level logical groupings of functional or topical operations that form the main building blocks within the architectural framework.