Source : Webster's Revised Unabridged Dictionary (1913)

Knight service \Knight service\ Also Knight's service \Knight's
service\
   1. (Feud. Law) The military service by rendering which a
      knight held his lands; also, the tenure of lands held on
      condition of performing military service.

            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.

   2. Service such as a knight can or should render; hence, good
      or valuable service.

Source : WordNet®

knight's service
     n : land tenure by service to the lord as a knight