[deadbeeves]

I'll take a stab at it: a multicellular organism dies when the processes that are able to sustain the operation of the cell community cease, thus causing the individual cells to die individually or to live as single-celled organisms in the wild for a short while. The main processes that are essential to the life of an organism as a whole are the supply of nutrients and oxygen, the elimination of waste, and the coordination of various support activities between different groups of cells in the body. They're essential because most of the cells have delegated the capacity to perform these functions to the organism, allowing an increase in efficiency at the cost of tying their survival to the survival of the organism.

A cell dies when it's damaged mechanically or chemically, or when it's unable to sustain its metabolism thus causing chemical damage, or when its programming instructs it to destroy itself for whatever reason.