Final answer:
Athetosis is the term describing involuntary muscle movements, characterized by slow, writhing movements, unlike ataxia which involves a lack of voluntary muscle coordination.
Step-by-step explanation:
The term used to describe involuntary muscle movements is C. Athetosis. This type of movement disorder is characterized by slow, involuntary, convoluted, writhing movements of the fingers, hands, arms, and sometimes the toes and feet. It's typically associated with certain neurological disorders and different from ataxia, which relates to voluntary muscle control.
On the other hand, ataxia is a condition related to a lack of muscle coordination during voluntary movements and may involve the cerebellum, which is a region of the brain that regulates coordination and balance.
Conditions like flaccidity describe a lack of muscle tone, and vestibular function is related to balance and spatial orientation, driven by the vestibular system within the inner ear and brain.