What is a Definition?

A definition is a way to partition the universe of discourse into two disjoint classes, one class containing all entities subsumed by the definition, the other class containing all entities excluded by the definition.

Template for a definition: An X is a Y, qualified by Z.

Example: A wife [X] is a woman [Y] who is married [Z].

This template suggests that the universe is naturally organized as a tree, and indeed, if there is a clear superclass Y, it should be used. Complex relationships may be expressed either through a complex superclass Y or by a complex qualifier Z, or both.

The following table exemplifies the X is a Y relationship for a number of terms of interest, mostly from ITU-T G.800 and G.805. For example, a path layer network is a form of layer network, which is a form of topological component, which is a form of architectural component.

Architectural component

  • Topological component
    • Layer network
      • Path layer network
    • Link
      • Topological link (MTOSI)
    • Server subnetwork
    • Subnetwork
    • Transitional link
  • Transport entity
    • Connection
    • Link connection
    • Network connection
    • Network transport entity
    • Subnetwork connection
    • Subnetwork transport entity
    • Trail
      • Path
      • Section
    • Unidirectional connection
    • Unidirectional trail
  • Reference point
    • Access point
    • Connection point
    • Termination connection point
    • Unidirectional access point
    • Unidirectional connection point
    • Unidirectional termination connection point
  • Transport processing function
    • Adaptation
    • Adaptation sink
    • Adaptation source
    • Forwarding function
    • Layer processor function
    • Trail termination
    • Trail termination sink
    • Trail termination source


  • Applications/clients (3GPP)
  • Bearer service (3GPP)
  • Connectionless service
  • Constant bit rate service
  • Customer-facing service (MTOSI)
  • Distribution service
  • Guaranteed service
  • Interactive service
    • Messaging service
    • Conversational service
    • Retrieval service
  • Multimedia service
  • Point to multipoint service
    • Multicast service
  • Point to point service
  • Resource-facing service
  • Sound retrieval service
  • Variable bit rate service
  • Video messaging


  • Telecommunication port (MTOSI)
  • Unidirectional port

Protection type

  • Subnetwork connection protection
  • Trail protection