Terminal illness or end-stage disease is a disease that cannot be cured or adequately treated and is expected to result in the death of the patient. This term is more commonly used for progressive diseases such as cancer, dementia or advanced heart disease than for injury. In popular use, it indicates a disease that will 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 as being terminal. There is no standardized life expectancy for a patient to be considered terminal, although it is generally months or less. Life expectancy for terminal patients is a rough estimate given by the physician based on previous data and does not always reflect true longevity. An illness which is lifelong but not fatal is a chronic condition.

Terminal patients have options for disease management after diagnosis. Examples include caregiving, continued treatment, palliative and hospice care, and physician-assisted suicide. Decisions regarding management are made by the patient and their family, although medical professionals may offer recommendations of services available to terminal patients.

Lifestyle after diagnosis varies depending on management decisions and the nature of the disease, and there may be restrictions depending on the condition of the patient. Terminal patients may experience depression or anxiety associated with impending death, and family and caregivers may struggle with psychological burdens. Psychotherapeutic interventions may alleviate some of these burdens, and is often incorporated into palliative care.

Because terminal patients are aware of their impending deaths, they have time to prepare for care, such as advance directives and living wills, which have been shown to improve end-of-life care. While death cannot be avoided, patients can strive to die a death seen as good.