What is Terminal Illness?

Legal Definition
Terminal illness is a disease that cannot be cured or adequately treated and that is reasonably expected to result in the death of the patient within a short period of time. This term is more commonly used for progressive diseases such as cancer or advanced heart disease than for trauma. In popular use, it indicates a disease that will soon progress until death with near absolute certainty, regardless of treatment.

A patient who has such an illness may be referred to as a terminal patient, terminally ill or simply terminal. Often, a patient is considered terminally ill when their estimated life expectancy is six months or less, under the assumption that the disease will run its normal course. The six-month standard is arbitrary, and best available estimates of longevity may be incorrect. Consequently, though a given patient may properly be considered terminal, this is not a guarantee that the patient will die within six months. Similarly, a patient with a slowly progressing disease, such as AIDS, may not be considered terminally ill because the best estimates of longevity were greater than six months. However, this does not guarantee that the patient will not die unexpectedly early. In general, physicians slightly overestimate the survival time of terminally ill cancer patients, so that, for example, a person who is expected to live for about six weeks would likely die around four weeks.
-- Wikipedia
Legal Definition
a term that is applied to an incurable illness that will end in a person's death.