The Collaborative International Dictionary of English v.0.48:

Discoid \Dis"coid\, a. [Gr. ? quoit-shaped, ? a round plate,
   quoit + e'i^dos form, shape: cf. F. disco["i]de. See Disk.]
   Having the form of a disk, as those univalve shells which
   have the whorls in one plane, so as to form a disk, as the
   pearly nautilus.
   [1913 Webster]

   Discoid flower (Bot.), a compound flower, consisting of
      tubular florets only, as a tansy, lacking the rays which
      are seen in the daisy and sunflower.
      [1913 Webster]