IP addresses assigned to an internal host by an enterprise. They are only used in private networks and cannot access the internet because they are not unique. Identical RFC1918 IPs are used on many different isolated networks. To ensure this strategy works, all internet routers are configured to discard IP packets with RFC1918 IPs.
Why we need RFC1918
When it became clear that IPv4 addresses were not going to be enough to support internet growth, RFC1918 was developed. It allowed private entities to use the same IP addresses internally that other organizations use as long as they did not have access to the internet. The only way for a device with an RFC1918 IP address to connect to the internet is to use an internal host as a middle man.