Final answer:
The term for the gelatinous cap surrounding the hair cells within the ampullae of the semicircular ducts is the cupula.
Step-by-step explanation:
The gelatinous cap surrounding the stereocilia and kinocilium of the hair cells within the ampullae of the semicircular ducts is referred to as the cupula. The ampulla is the structure at the base of each semicircular canal that contains the hair cells and cupula necessary for the transduction of rotational movements of the head. The vestibular system utilizes these structures to sense head position and movement, aiding in balance and spatial orientation.