• (n.) That part of a bird, or the feathers, surrounding the cloacal opening; the under tail coverts.
Area of feathers between the vent and rectrices that includes the undertail coverts.
Cris'sum (kr?s's?m),, noun ; plural Crissa (-s...). [ New Latin ; confer Latin crisso to move the haunches.] (Zoology) That part of a bird, or the feathers, surrounding the cloacal opening; the under tail coverts. 
triangular area of feathers on the undersurface of a bird between its vent (anal opening) and the base of its tail feathers.
also called the undertail coverts, the contour feathers behind the vent covering the bases of the retrices. 
The feathers in a triangular area on the underside of a bird between its vent and the base of its tail feathers.
the region surrounding the cloacal opening beneath the tail of a bird. · the feathers of this region collectively.