(by extension) One who sacrifices his or her life, station, or something of great personal value, for the sake of principle or to sustain a cause.
(with a prepositional phrase of cause) One who suffers greatly and/or constantly, even involuntarily.
Verb
(transitive) To make someone into a martyr by putting him or her to death for adhering to, or acting in accordance with, some belief, especially religious; to sacrifice on account of faith or profession.
