Protected information system link utilizing tunneling, security controls, and endpoint address translation giving the impression of a dedicated line.
Sources:
CNSSI 4009-2022

  Restricted-use logical computer network that is constructed from the system resources of a physical network by using encryption and/or by tunneling links of the virtual network across the real network.
Sources:
CNSSI 4009-2022 from IETF RFC 4949 Ver 2 - adapted

  Protected information system link utilizing tunneling, security controls, and endpoint address translation giving the impression of a dedicated line.
Sources:
NIST SP 1800-21B under Virtual Private Network from CNSSI 4009-2022
NIST SP 800-53 Rev. 5 under virtual private network from CNSSI 4009-2022

  A virtual network built on top of existing networks that can provide a secure communications mechanism for data and IP information transmitted between networks.
Sources:
NIST SP 800-113 under Virtual Private Network

  A virtual network built on top of existing physical networks that can provide a secure communications mechanism for data and IP information transmitted between networks or between different nodes on the same network.
Sources:
NIST SP 800-77 Rev. 1 under Virtual Private Network (VPN)

  A restricted-use, logical (i.e., artificial or simulated) computer network that is constructed from the system resources of a relatively public, physical (i.e., real) network (such as the Internet), often by using encryption (located at hosts or gateways), and often by tunneling links of the virtual network across the real network.
Sources:
NIST SP 800-82r3 under virtual private network from RFC 4949 - adapted