Definition from Wiktionary, the free dictionary
apterium (plural apteria)
- (ornithology) An area of bare (featherless) skin on a bird.
1918, American Ornithologists' Union, The Auk:
- The two halves of the lower cervical tract are still separated by an apterium 4 mm. wide, as in the young bird, but contour feathers are now present on the chin and upper throat, so that they are united at their upper ends.
an area of featherless skin