What is it called when rabbits eat their feces?

Rabbits don't really eat their feces; they eat their cecotropes. This natural process is called "cecotrophy" (also sometimes spelled "caecotrophy"). Some people call "coprophagy," but this isn't technically correct as "coprophagy" refers to the eating of feces, but cecotropes (also known as cecal pellets or "night feces") are not technically feces because they aren't waste matter: rather, they're an important part of a rabbit's digestive system. Cecotrophy is a way for rabbits to redigest their food in order to get the most nutritional benefits out of it (in principle, cecotrophy in rabbits is comparable to chewing the cud in cows).