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, rather than fatal 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. An illness which will result in premature death, even if that death may be many years away, is called a life-limiting illness. An illness which is lifelong but not life-shortening is called 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.

When 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. However, many healthcare providers are uncomfortable telling people or their families that they are dying. To avoid uncomfortable conversations, they will withhold information and evade questions.

Accurately identifying the start of terminal status is important because it usually occasions a review of treatment goals. Although there is no single official definition, there are four typical characteristics for determining whether a person has a terminal illness:

When the remaining lifespan is expected to be days and the physical process of dying has begun, the term active dying may be used instead.

Most terminally ill people do not experience additional distress from honest prognosis discussions beyond their existing distress from the illness itself, and they usually value knowing whether their realistic lifespan is likely to be "weeks", "months", or "years", even if more specific estimates are unavailable. However, many healthcare providers avoid telling them this because the healthcare providers are uncomfortable with death or perceive it as a professional failure. To avoid admitting that the person will inevitably die from an incurable condition, they may withhold information or, if pressed, give overly optimistic answers. For example, if the typical person in that situation usually lives for two to six months, they may say only the larger number. They may rationalize the inflated claim by thinking of hopeful possibilities, such as an unproven treatment (which might shorten the person's life even further) being attempted, or because they know that life expectancy is an imperfect estimate and could be both shorter or longer than expected.

They may feel pressure from family members to give pleasant news or to preserve the false appearance of hope. They often want to avoid the emotional outbursts that are associated with people understanding the medical situation accurately. For example, they will use death-denying language such as "She has a life-limiting diagnosis" – a term that makes the inevitable death seem less inevitable – rather than bluntly saying "No matter what we do, your daughter is almost certainly going to die from this cancer, probably within the next few months."