I don't remember the exact definition I was originally taught, but I always kinda ran with something like "abnormality to an extent that it causes dysfunction in everyday life". Abnormality of course, defined according to ones own present sociocultural environment and and dysfunction given presumed inherent level of functioning.

I actually feel that the second half seems to be ignored more and more frequently, but that is what I have often been taught as an essential part of defining an actual state as being a disorder... versus just quirks, to be perfectly honest.

My opinion, which is not always shared and which I'm sure some people will take offense at- but please be assured I am not actually talking about anyone specifically on PsychCentral[ I have no right to, as this is the internet. I don't know what goes on in people's lives]... is that there is a huge increase of diagnosis for diagnosis sake.

I'm not entirely sure why?
With the release of the DSM 5, a lot of people- including some who actually worked on previous publications of the DSM- have voiced specific and very public concerns regarding the expansion of the number of people who may be included in this notion of "mental illness".

I have simlar concerns as well, but then i'm starting to go off topic so I'll leave it at that. :P