Membranous, as a chorion; like or likened to the chorion, as an investing part or tunic: in anatomy, applied to several delicate, highly vascular membranes which invest certain parts, and to associated structures.n. A delicate, highly vascular membrane forming one of the coats or tunics of the eyeball, lining the sclerotic, and lying between it and the retina, with which it is in contact by its inner surface.