Well, there isn't one universal definition, but definitions of mental health usually encompass the idea that people can have successful relations with other people, that they can make contributions to society by holding down a job. How they think and how they feel allows them to function fully and engage in everyday life.
When you start talking about mental illness, then you're really talking about having how you think, how you feel, how you respond, and how you behave affected by your illness. So the symptoms are manifested through, as I say, how you think, how you feel, or how you behave.