The Collaborative International Dictionary
Knight service \Knight" serv"ice\Also Knight's service \Knight's service\n.
(Feud. Law) The military service by rendering which a knight held his lands.
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(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.
--Pollock & Mait. Service such as a knight can or should render; hence, good or valuable service.
Wiktionary
alt. (context now historical English) The military service a knight owed under the feudal system as a condition of holding his lands; hence, the system of land tenure based on such service. n. (context now historical English) The military service a knight owed under the feudal system as a condition of holding his lands; hence, the system of land tenure based on such service.
Usage examples of "knight service".
It would be doing me knight service if you keep your eyes fixed upon them, and remember them every minute of the day, and tell me directly I drop one.
I have given lands and privileges to get knight service, and kept my army in the field by contracts my forefathers never countenanced.