VPN

Protected information system link utilizing tunneling, security controls, and endpoint address translation giving the impression of a dedicated line. 

See NIST SP 800-53 Rev. 4 under Virtual Private Network (CNSSI 4009) for more information. 
 
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. 

See NIST SP 800-82 Rev. 2 under Virtual Private Network (VPN) (RFC 4949) for more information. 
 
A data network that enables two or more parties to communicate securely across a public network by creating a private connection, or “tunnel,” between them. 

See NIST SP 800-47 under Virtual Private Network (VPN) for more information. 
 
A virtual network built on top of existing networks that can provide a secure communications mechanism for data and IP information transmitted between networks. 

See the following under Virtual Private Network for more information. 

NIST SP 800-113. 
NIST SP 800-77. 
 
A logical network that is established at the network layer of the OSI model. The logical network typically provides authentication and data confidentiality services for some subset of a larger physical network. 

See NIST SP 800-127 under Virtual private network (VPN) for more information.