A Node in a domain is
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Logical representation of machine
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collection of domains
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Distributed set of administrative services
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Distribute set of application services and Service Manger
In network domains (specifically Windows Active Directory or similar directory services), a node represents a single machine or computer account within the domain structure. It's the logical representation of that physical or virtual machine's identity and membership in the domain.
In networking/domain terminology, a node represents a single logical machine (a computer, device, or endpoint) participating in the domain — hence "logical representation of machine" is correct. A collection of domains would instead be a forest or tree, not a node. "Distributed set of administrative services" describes the domain itself (or a domain controller's role), not an individual node. "Distributed set of application services and Service Manager" describes a server or service architecture concept, not the basic unit a node represents. The node is the smallest addressable entity — one machine — within the domain hierarchy.