1. a health care profession that focuses on the protection and promotion of health through the alleviation and treatment of illness, injury, disease, and physical suffering. Nurses practice in a variety of contexts, including hospitals, nursing and independent-living homes, schools, workplaces, and community centers, among others. In the United States, nurses must graduate from a state-approved school of nursing (a 4-year university program, a 2-year associate degree program, or a 3-year diploma program) and pass a state licensing examination. See also licensed practical nurse; registered nurse.

2. the provision of nourishment by a female for her young offspring until they are capable of obtaining their own food. Nursing in mammals (including humans, in whom it is called breast-feeding) primarily involves the secretion of milk from the mammary glands, as stimulated by the hormones prolactin and oxytocin. Other vertebrates exhibit different forms of nursing behavior. For example, some birds produce a milklike substance (crop milk) within their digestive system that is regurgitated to feed young chicks. The length of the nursing period varies across animals, ranging from mere days (e.g., Sprague–Dawley rats) to several years (e.g., bottlenose dolphins). Regardless of its form or duration, however, successful nursing is critical to survival and often depends on maternally emitted odorants (mammary pheromones) to enable the young to locate the nipple and initiate suckling.