Health, in a living organism, can be understood as homeostasis - the state in which the organism is in balance, with inputs of energy and mass and outputs of energy and mass in approximate equilibrium (minus whatever mass is retained in the normal growth process), and the prospects for continued survival of the organism are positive.
In humans, with the capacity to analyze and anticipate, health can be understood not only as immediate homeostasis - that is, everything being all right at the moment - but also subjectively as the understanding of the potential of the "healthful" balance being able to continue. The study of health of humans and animals is health science. Doctors, dentists, and biologists are health scientists.
This understanding comes from somatic perception, including pain and discomfort, as well as cognitive perception, such as one's knowing that they look well, are functioning as well as they always have, and knowing that no imminent external or internal risk endangers the healthful state.
In the late 20th century, the subjective perception of healthfulness in human beings came to be known as wellness.