The Collaborative International Dictionary of English v.0.48:

Knighthood \Knight"hood\, n. [Knight + hood: cf. AS. chihth[=a]d
   youth.]
   1. The character, dignity, or condition of a knight, or of
      knights as a class; hence, chivalry. "O shame to
      knighthood." --Shak.
      [1913 Webster]

            If you needs must write, write Caesar's praise;
            You 'll gain at least a knighthood, or the bays.
                                                  --Pope.
      [1913 Webster]

   2. The whole body of knights.
      [1913 Webster]

            The knighthood nowadays are nothing like the
            knighthood of old time.               --Chapman.
      [1913 Webster]

   Note: "When the order of knighthood was conferred with full
         solemnity in the leisure of a court or court or city,
         imposing preliminary ceremonies were required of the
         candidate. He prepared himself by prayer and fasting,
         watched his arms at night in a chapel, and was then
         admitted with the performance of religious rites.
         Knighthood was conferred by the accolade, which, from
         the derivation of the name, would appear to have been
         originally an embrace; but afterward consisted, as it
         still does, in a blow of the flat of a sword on the
         back of the kneeling candidate." --Brande & C.
         [1913 Webster]

WordNet (r) 3.0 (2006):

knighthood
    n 1: aristocrats holding the rank of knight

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "knighthood":
   arms, art of war, baronetcy, barony, chivalry, dukedom, earldom,
   generalship, kingship, knight-errantship, ladyship, lordship,
   marquisate, pashadom, princedom, princeship, queenship,
   seignioralty, seigniory, viscountcy, viscountship, viscounty,
   war