What is osteonecrosis of the jaw?

A drug side-effect: The drugs for osteoporosis and metastatic bone cancer called bisphosphonates (includes fosamax, actonel, boniva, reclast, (zoledronic acid) zometa, aredia) are associated with a side-effect where part of the bone of the jaw dies, the gum sloughs off, and bone shows where the gum was lost. It is treatable but not always curable. It has also been found with bevicizumab, an anti-cancer drug, and after jaw radiation.

The: Mandible starts to die for various reasons to cause no blood flow.

Tissue death: Dr. Hinshaw's answer is quite correct regarding osteonecrosis secondary to medication. It is important to know that osteonecrosis is not confined to the jaw, but rather any bone that continues to lose blood supply and vitality. Certain bacteria are implicated as well. Cutting away dead tissue is a must, and sometimes a patient needs to be put in a hyperbaric chamber (high pressure oxygen). Serious.

Dead bone: Dead bone in jaw. Usually due to radiation treatment to jaw for cancer. or bone building medications. Serious problem. See oral surgeon for exam/diagnosis/treatment.

Osteonecrosis jaw: Osteonecrosis of the jaw, commonly called ONJ, occurs when the jaw bone is exposed and begins to starve from a lack of blood. http://www.rheumatology.org/I-Am-A/Patient-Caregiver/Diseases-Conditions/Osteonecrosis-of-the-Jaw-ONJ