Die: "To pass from an animate to a lifeless state; to cease to live; to suffer a total and irreparable loss of action of the vital functions; to become dead; to expire; to perish..."
This definition for "die" is from the 1913 edition of Webster's Dictionary. Notice that dying is a process. It has a beginning, a middle, and an end. Some people might say the beginning is right after we are born. Others may say dying begins when we stop growing, about our late teens to early 20's. Still, others may say it begins when we contract a physical condition (e.g., illness, or physical injury) from which there is no possible recovery. No matter what you consider the start of dying to be, throughout our lives cells within us are in a race. It is a race between cells ceasing to live and cells being created. When we are young, new cells are winning the race - we generate cells more quickly than others die, we generate bone cells in greater numbers than the number of bone cells that die off. When growth ends, the race is even - the rate of cell generation slows down to a maintenance and replacement level. We generate cells at nearly the same rate as they are needed to replace dying cells. Over time, the rate of cell generation slows and the rate of cell degeneration increases; we age, i.e., we're "losing the race." We realize that we move more slowly, we don't heal as quickly, we don't see or hear as well, etc. Our bodies are wearing out. Add on the effects of physical injuries and diseases, and it is no wonder that we lose the ability to repair and recover. Eventually, the destruction is so much greater than the ability to generate that we label the condition "dying." The race is soon over and the process of dying ends. Physical death replaces physical life.
All living creatures die: plants, mammals, insects, fish, birds, etc., and humans. Humans are different, however, because we KNOW we can die, when we are dying, and what that means to our existence. We can comprehend the paradox of life vs. death, existence vs. nonexistence, and being vs. non-being. This section of my site discusses how we manage to exist in the face of our own, or of someone we love, dying.