Address Translation (NAT) and Port Address Translation (PAT)
Source: Global Knowledge
It's common today to use private addressing within an Autonomous System (AS), a collection of routers and subnets under a common administrative domain. The problem is that per RFC 1918 (Address Allocation for Private Internets), advertising these address spaces to the public Internet is not allowed. This means that if you send a packet with a "private" source address to the Internet, the destination will not be able to reply to you because the routers on the Internet backbone won't know where you are. The solution to this problem is Network Address Translation (NAT).