noun

• (historical, law) The grantee of a fief, a subordinate granted use of a superior's land and its income in exchange for vows of fidelity and homage and (typically) military service.
  "The manor's vassals owed first fruits and a tithe to the parish church, another 10% to the lord (including at least 50 eels), a week or two each year of service in the manor's upkeep, and service in the local fyrd."
• (historical) Any direct subordinate bound by such vows to a superior.
  "The king ordered his vassals to join him on the crusade unless they had a written note signed by the archbishop or pope."
• (figurative) Any subordinate bound by similar close ties.
  "The vassals of his anger."

verb

• (transitive) To treat as a vassal or to reduce to the position of a vassal; to subject to control; to enslave.
• (transitive) To subordinate to someone or something.