Final answer:
The system where individual stations purchase and air shows that were originally broadcast on network television is called off-network syndication.
Step-by-step explanation:
When a popular television show like Prison is purchased by individual stations and re-aired in different parts of the country, this system is known as off-network syndication. In this process, shows that were originally produced and broadcast for network television are sold to local stations or cable channels for reruns.
This is distinct from first-run syndications, where shows are broadcast for the first time not on a network, and network syndications, which involve the distribution of shows to network-affiliated stations.