\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}.