Webster’s Revised Unabridged Dictionary · 1913
Hypoptilum
Hy·pop′ti·lum (hī·pŏp′tĭ·lŭm), n.; pl. L. Hypoptila (#), E. Hypoptilums (#). [NL., fr. Gr. ὑπό beneath + πτίλον down.] (Zoöl.) 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.