Elaborate on the phrase "domain as a concept."

What will be an ideal response?

Domain is a concept, which means it is a perception of reality, not the reality itself – even though the perception might come very close to reality. Nevertheless, domains do display an inherent logical coherence once we discover the rules or, perhaps, once we select and organize them based on our preferences and our aims. Animal Kingdom is a recognizable domain: We distinguish between the rules that apply to a bear and those that apply to a rose, which belongs to the Plant Kingdom.
Domains, however, are not mutually exclusive. Within one domain, we can distinguish other domains and sub-domains: birds and mammals within the Animal Kingdom, for example. They may also overlap: we can take elements from one domain, combine it with elements from another one and come up with a new domain that is equally valid. For example, take a fly from the Insects domain, a finch from the Birds, and a bat from the Mammals and we are likely to "discover" a domain in which the rules of aerodynamics apply because they all fly. The new domain does have an internal coherence and an objective reality, but we arrive at it by an act of selection.