Medicine is a branch of health science and the sector of public life concerned with maintaining or restoring human health through the study, diagnosis, treatment and possible prevention of disease and injury. It is both an area of knowledge – a science of body systems, their diseases and treatment – and the applied practice of that knowledge

Western medical care is shared between medical professionals (physicians) and other professionals such as physician assistants, nurses and pharmacists, sometimes known as allied health professionals. Historically, only those with a medical doctorate have been considered to practice medicine. Clinicians (licensed professionals who deal with patients) can be physicians, nurses, therapists or others. The medical profession is the social and occupational structure of the group of people formally trained and authorized to apply medical knowledge. Many countries and legal jurisdictions have legal limitations on who may practice medicine.

Medicine comprises various specialized sub-branches, such as cardiology, pulmonology, neurology, or other fields such as sports medicine, research or public health.

Human societies have had various different systems of health care practice since at least the beginning of recorded history. Medicine, in the modern period, is the mainstream scientific tradition which developed in the Western world since the early Renaissance (around 1450). Many other traditions of health care are still practiced throughout the world; most of these are separate from Western medicine, which is also called biomedicine, allopathic medicine or the Hippocratic tradition. The most highly developed of these are traditional Chinese medicine, Tibetan medicine and the Ayurvedic traditions of India and Sri Lanka. Various non-mainstream traditions of health care have also developed in the Western world. These systems are sometimes considered companions to Hippocratic medicine, and sometimes are seen as competition to the Western tradition. Few of them have any scientific confirmation of their tenets, because if they did they would be brought into the fold of Western medicine.

"Medicine" is also often used amongst medical professionals as shorthand for internal medicine. Veterinary medicine is the practice of health care in animal species other than human beings

Medicine is practiced within the medical system, which is a legal, credentialing and financing framework, established by a particular culture or government. The characteristics of a health care system have significant effect on the way medical care is delivered.