Dead on arrival or D.O.A. is a term used primarily by non-medical emergency responders such as police and firefighters. It implies that the victim has "sustained injuries not conducive to supporting life", and that the the Emergency Medical Service dispatcher should advise the responding paramedic unit that the call will now be a "pronouncement" ( patient pronounced dead). The reason that paramedics must still respond is that legally the only individual who may officially pronounce a person dead is a physician, so the medics will still have to make an assesment of the patient. This includes an EKG which would reflect a lack of electrical activity in the heart, known as asystole, which would be tansmitted via cell phone or radio to the attending ER physician along with a report of the situation,who then in turn gives them a time of death for the individual. The medical examiner will then respond to remove the body.
It should be noted however that this term is not typically officially used due to the legal ramifications present should someone who is not medically trained state that a patient is dead only to find out that they could have been treated with the possibility of recovery. The only time someone other than a physician can officially pronounce someone dead is in the aforementioned "injuries not conducive with sustaining life" such as decapitation or during a mass casualty incident (MCI) during which multiple patients must be triaged and people lacking self sustained breathing are declared dead.