Source : Webster's Revised Unabridged Dictionary (1913)

Hypoptilum \Hy*pop"ti*lum\, n.; pl. L. {Hypoptila}, E.
   {Hypoptilums}. [NL., fr. Gr. "ypo` beneath + ? down.]
   (Zo["o]l.)
   An accessory plume arising from the posterior side of the
   stem of the contour feathers of many birds; -- called also
   {aftershaft}. See Illust. of {Feather}.