"Dead" refers to the state of an organism that has ceased all biological functions necessary for life. This includes the stopping of the heart, brain activity, and respiration. In humans and many animals, death can occur due to various reasons, such as illness, injury, or old age. In a broader context, the concept of being dead can also apply to non-living things, such as dead cells in the body or dead languages that are no longer spoken. The study of death and its implications is explored in fields like thanatology and mortuary science.