Any deceased creature which has been animated through supernatural means to take on the semblance of life, without truly being alive. Only the wraith varies from this definition as it need not pass though death to become undead.
A person that was near death and has been saved by medicine.
Going through undead state is also known as a near death experience.
John got into a car accident and was in a coma for 40 days. The doctor's told John's parents that it was highly unlikely for him to live. Different techniques and medicines made it possible for John to live, so now he is undead.