Hy*pop"ti*lum (?), n.; pl. L. Hypoptila (#), E. Hypoptilums (#). [NL., fr. Gr. 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.
© Webster 1913.