1.
retainer, vassal
A person who serves a lord or feudal ruler. In Japanese history, refers to the samurai who served under a daimyō or other feudal lord.
Usage
Primarily a historical term from the feudal era. 家臣 specifically implies a hereditary or household retainer — someone who belongs to the lord's household as opposed to a temporary ally. The related term 家臣団 refers to the collective body of retainers.
In modern usage, 家臣 appears mainly in historical fiction, period dramas, and discussions of Japanese history.