(Feud. Law) The military service by rendering which a knight held his lands.
2.
(Eng. Feud. Law) A tenure of lands held by knights on condition of performing military service. See Chivalry, n., 4. "By far the greater part of England (in the 13th century) is held of the king by knight's service.... In order to understand this tenure we must form the conception of a unit of military service. That unit seems to be the service of one knight or fully armed horseman (servitium unius militis) to be done to the king in his army for forty days in the year, if it be called for.... The limit of forty days seems to have existed rather in theory than practice."
3.
Service such as a knight can or should render; hence, good or valuable service.
... wound given to the feudal system was the Act of the 12th of Charles II, which abolished the tenure of knight's service in capite, and which Blackstone compares, for its salutary influence upon property, to the boasted provisions of Magna Charta itself. Yet even in this act we see the effects of that counteracting spirit which ... — The Complete Poems of Sir Thomas Moore • Thomas Moore et al