The Collaborative International Dictionary of English v.0.48:

Hypoptilum \Hy*pop"ti*lum\, n.; pl. L. Hypoptila, E.
   Hypoptilums. [NL., fr. Gr. "ypo` beneath + ? down.] (Zool.)
   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.
   [1913 Webster]