What is an abstract noun for serf?

Some people would say slave, but that is not really correct because a slave could be sold, and a serf could not. Also, a slave had to do whatever told, but a serf simply had to fulfill obligatory duties.

A villein was very like a serf, except that a villein lived in town, but a serf was agricultural.

A cotter was very like a serf, but we really do not have other information on what a cotter was. It is thought that the cotter was technically free to leave the land, but this is speculation.