A program is confined when it cannot transmit information to any program other than the one who called it.
The confinement problem is the problem of enforcing the execution of a program to be confined.
The notion of confinement was introduced by Bruce Lampson in a 1973 CACM paper. The paper was also the first one to introduce the notion of covert channel along with examples of such channels.
Confinement is closely related to various modern notions such as Unix and BSD jails, sandboxing, containers, virtualization, VMs and hypervisors, etc.
Butler W. Lampson. 1973. A note on the confinement problem. Commun. ACM 16, 10 (Oct. 1973), 613–615. https://doi.org/10.1145/362375.362389
@article{10.1145/362375.362389,
author = {Lampson, Butler W.},
title = {A Note on the Confinement Problem},
year = {1973},
publisher = {Association for Computing Machinery},
address = {New York, NY, USA},
volume = {16},
number = {10},
doi = {10.1145/362375.362389},
journal = {Communications of the ACM},
pages = {613–615}
}