Final answer:
Aging is the time-dependent process involving increased functional loss and susceptibility to disease and death, studied within thanatology.
Step-by-step explanation:
The time-dependent biological process that involves functional loss and susceptibility to disease and death is called aging. As people age, they may experience increased frailty and a higher risk of diseases such as Alzheimer's, which is characterized by the accumulation of amyloid plaques in the brain—a condition known as proteopathy. The processes of aging and death, including the different stages of grief and the concept of hospice care, are crucial aspects of thanatology, the study of death and dying.