The Collaborative International Dictionary of English v.0.48:

Knight service \Knight" serv"ice\Also Knight's service \Knight's
service\n.
   1. (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.
      [1913 Webster]

            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.
      [Webster 1913 Suppl.]

   3. Service such as a knight can or should render; hence, good
      or valuable service.
      [Webster 1913 Suppl.]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

KNIGHT'S SERVICE, Eng. law. It was, formerly, a tenure of lands. Those who
held by knight's service were called: milites qui per loricas terras suas
defendunt;: soldiers who defend the country by their armor. The incidents of
knight's service were. homage, fealty, warranty, wardship, marriage,
reliefs, heriots, aids, escheats, and forfeiture. Vide Socage.