I always thought of death as the transition from a living to a non-living state. That's more of a academic view on the event. You may be looking for a more spiritual answer. I will post the dictionary link in case you want to browse it.
Death is what you want it to be, reason I say is because there are people in this world that choose not to live, based on there ill will to live or lock them selves up from the world. When we stop breathing and never wake up to see the sun light peeking thru our windows, now that's when you know your dead.
Death is the end of life, a permanent cessation of all vital functions. Dying refers to the body's preparation for death, which may be very short in the case of accidental death, or can last weeks or months.