What is the definition of health?

There is no acclamation, actually.
First according to WHO

Health is a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity.

But, Rudolfo Saracci,director of research in epidemiology at National Research Council, Pisa, Italy wrote:

The World Health Organisation's definition of health seems to work against its effective functioning. When the WHO was established nearly half a century ago the text of its constitution defined health as "a state of complete physical, mental and social well being and not merely the absence of disease or infirmity."
This by now classical definition of health, conceived in the aftermath of the second world war, when peace and health were seen as inseparable,had one merit lasting long beyond the circumstances of origin: it made explicit that disease and infirmity, when isolated from subjective experience, are inadequate to qualify health. Widening health to the psychological and the social dimension was a major advance, but although it was conceptually important, it had no direct operational value. As one commentator put it, "This [definition] is a fine and inspiring concept and its pursuit guarantees health professionals unlimited opportunities for work in the future, but is not of much practical use.