Feudalism
noun
You know that 主義 is "-ism." Add that onto an ism that is building a seal(ed) in system, and you have feudalism. Why? Because with feudalism, power is sealed in with one group or one family. Nobody else can get in (until they are overthrown, anyways), making this feudalism.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
今日は歴史の授業で封建主義について学びました。
Today we learned about feudalism in history class.