Acronym for Network Address Port Translation.
A derivative of NAT, NAPT is all about taking incoming requests to an IP address and rewriting them to all go to a single IP address and port, based on what port they are aimed at. This leads to the common name of PinHoling (you poke a pin hole in your firewall, allowing all access on a certain port to statically map to a (potentially different) port on an internet machine, such as a public web or email server.
NAPT is defined in 
RFC:3022.
Because NAPT deals with ports, it only applies to TCP, UDP and ICMP.
