hypoptilum

from The Century Dictionary.

• noun In ornithology, the aftershaft or hyporachis of a feather; the whole of the supplementary plume which springs from the stem of the main feather at the junction of calamus and rachis.

from the GNU version of the Collaborative International Dictionary of English.

• noun (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun ornithology An aftershaft.