Sometimes, mental illness is brought into focus by what it is not. The World Health Organization defines health as: " ... a state of complete physical, mental, and social well-being and not merely the absence of disease or infirmity."

WHO states:

Mental Health refers to a broad array of activities directly or indirectly related to the mental well-being component included in the WHO's definition of health: "A state of complete physical, mental and social well-being, and not merely the absence of disease". It is related to the promotion of well-being, the prevention of mental disorders, and the treatment and rehabilitation of people affected by mental disorders.

More precisely:

Q: What is mental health?

A: Mental health is not just the absence of mental disorder. It is defined as a state of well-being in which every individual realizes his or her own potential, can cope with the normal stresses of life, can work productively and fruitfully, and is able to make a contribution to her or his community. http://www.who.int/features/qa/62/en/index.html

Mental illness in the broader sense then is an impediment that stands in the way of our reaching our potential, coping with normal stresses, working productively and fruitfully and from making a contribution to our communities.