The Collaborative International Dictionary of English v.0.48:

Discus \Dis"cus\, n.; pl. E. Discuses, L. Disci. [L. See
   Disk.]
   1.
      (a) A quoit; a circular plate of some heavy material
          intended to be pitched or hurled as a trial of
          strength and skill.
      (b) The exercise with the discus.
          [1913 Webster]

   Note: This among the Greeks was one of the chief gymnastic
         exercises and was included in the Pentathlon (the
         contest of the five exercises). The chief contest was
         that of throwing the discus to the greatest possible
         distance.
         [1913 Webster]

   2. A disk. See Disk.
      [1913 Webster]

WordNet (r) 3.0 (2006):

discus
    n 1: an athletic competition in which a disk-shaped object is
         thrown as far as possible
    2: a disk used in throwing competitions [syn: discus,
       saucer]

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "discus":
   O, annular muscle, annulus, areola, aureole, ball, chaplet, circle,
   circuit, circumference, circus, closed circle, corona, coronet,
   crown, cycle, diadem, disk, ejecta, ejectamenta, eternal return,
   fairy ring, garland, glory, halo, lasso, logical circle, loop,
   looplet, magic circle, missile, noose, orbit, projectile, quoit,
   radius, ring, rondelle, round, roundel, saucer, sphincter,
   trajectile, vicious circle, wheel, wreath