The term ‘mental illness’ is generally used when someone experiences significant changes in their thinking, feelings or behaviour. The changes need to be bad enough to affect how the person functions or to cause distress to them or to other people.
The terms ‘ ‘mental health problem’ and mental disorder’ have a similar meaning.
If a person has always had a problem in their thinking, feeling or behaviour, then this is not usually called mental illness. It may then be called a developmental problem or a difficulty with their personality (sometimes called a personality disorder).
Mental health is the opposite – it means mental wellbeing, good mental functioning or having no particular problems in thinking, feelings or behaviour.
These kinds of definitions of course greatly over-simplify things. All of us experience changes from time to time in our feelings, thinking and behaviour, and there is no clear cut off between illness and health. Also someone may have problems which fit the definition of a mental illness, but they may be very healthy mentally in other ways.