Network Address Translation (NAT) – Glossary | CSRC

  A routing technology used by many firewalls to hide internal system addresses from an external network through use of an addressing schema.
Source(s):

NIST SP 800-41 Rev. 1

  The process of mapping addresses on one network to addresses on another network.
Source(s):

NIST SP 800-86
under Network Address Translation

  A function by which internet protocol addresses within a packet are replaced with different IP addresses. This function is most commonly performed by either routers or firewalls. It enables private IP networks that use unregistered IP addresses to connect to the internet. NAT operates on a router, usually connecting two networks together, and translates the private (not globally unique) addresses in the internal network into legal addresses before packets are forwarded to another network.
Source(s):

NIST SP 1800-15B

  A function by which internet protocol (IP) addresses within a packet are replaced with different IP addresses. This function is most commonly performed by either routers or firewalls. It enables private IP networks that use unregistered IP addresses to connect to the internet. NAT operates on a router, usually connecting two networks together, and translates the private (not globally unique) addresses in the internal network into legal addresses before packets are forwarded to another network.
Source(s):

NIST SP 1800-15C