I think may be a lot of people are overreacting to what Prudence said about personality disorder.
I always understood personality disorder to be much milder than mental illness. People with personality disorders function perfectly well in the community and more often than not require no medication or psychiatric intervention, as is normally the case with mental illness. I believe the full term is "borderline personality disorder", which means the person functions perfectly normally except in one or two areas where they have reactions which seem unreasonable and/or extreme to their family and friends.
If you look at the explanatory link to the term "personality disorder" in Prudence's answer, it is clearly stated that around 15% of people have a personality disorder of some sort. I am darn certain 15% of the population are not mentally ill.
Perhaps it would have been better if Prudie and the rest of us who used the term "personality disorder" with regard to this stepdaughter had inserted the word "borderline" in front! However, it is not, and never has been, another way of saying "mental illness"