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 once living person, who is made undead without passing through death. A wraith is the only form of undead not made so through animation (the process of giving lifelike qualities to the recently dead).
"They are ringwraiths, neither living nor dead..."
Aragorn - Lord of the Rings, Fellowship of the Rign