There are different definitions of severe mental illness. However, in general severe mental illness can be defined a emotional distress or psychological symptoms impacting the person's ability to function, such as care for themselves regularly and maintain employment or relationships.

Functional level impacted at high level

Hello and thank you for your question. A severe mental illness is one which effects a person mood, thoughts and behaviors; ultimately making it difficult for him/her to take care of their daily basic needs. Many people have depression and it is treated with no issues. I hope this answers your question.
Warmest Regards

No, it does not count. It is about the severity of the illness and the ability to function. 

Most "mental illnesses" are in a continuum - there are mild cases and there are severe cases. A lot of people (50% of the population will at some point suffer from a form of "mental illness" in their lifetime) suffer from depression and after a while (3 to 6 months) the depression goes away. The severe case is when it does not go away, and causes many issues in other parts of their life: such as work, relationship, or even development of psychosis). Other severe mental illness is like what you'd see in people suffering from schizophrenia, or other debilitating illness.

Be well,

Dr. Rosana Marzullo-Dove

No that doesn’t count. Basically schizophrenia is a severe mental issue, where people hear voices and their mind is warped in believing things that are not happening or real

This link will help you address this question:

http://www.bhevolution.org/public/severe_mental_illness.page