A Punishment Loop is a stable branch (or variant of an existing loop) known for being very boring and highly unpleasant to be in. They are, as the name suggests, used as punishments for loopers who give their admins extra work. These loops lock down on all non-native Looper abilities, including subspace pockets, and usually force Loopers to follow the canon plot. They're very hard to crash, although the MLE Billy has managed to do so three times.
The most famous and often-used punishment loop is Eiken, but it isn't the only one. Teletubbies is also used, and the Equestrian loopers sometimes experience Generation 3 loops as an alternative. Caboose has Ren and Stimpy as his punishment loop, as he enjoys Eiken loops.