The Collaborative International Dictionary
Hypoptilum
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.
Wiktionary
hypoptilum
n. (context ornithology English) An aftershaft.