Maybe, unfortunately, I lent my copy of "Russia's First Civil War" to my brother and cannot look it up, though I'm pretty sure the author uses the term slave. It was during this time, early 1600's, that those who had sold themselves into slavery were made into permanent slave, essentially enserfing the Russian peasants at a time the rest of Europe had freed their serfs.
http://www.psupress.org/books/titles/0-271-02074-1.html