Short for Network Address Translation.
IP trick used to hide the details of a host or network behind a front-end machine, typically a firewall.
Also used to multiplex a single IP address (provided by an internet service provide), allowing the IP address to be shared by a small network (such as a home network or small business network). Some ISPs charge a lot more to dedicate a range of IP addresses instead of a single one, and NAT lets you get by with just a single one.