Failure to function adequately
There are three ways to define what abnormality is. The second is called failure to function adequately.
This definition refers to a person who cannot carry out behaviours that are expected by society and are seen as everyday activities e.g. getting out of bed each day.
The Diagnostic and Statistical Manual of Mental Health (DSM) is used by doctors to assess individuals on physical and psychological measures.
One such measure is the Global Assessment of Functioning Scale (GAF). A score of 100 indicates very positive functioning and interest in a wide range of everyday activities, whereas a score of 10 indicates there is a persistent danger of hurting one's self or others and an inability to commit to small tasks such as minimal hygiene, often accompanied by suicidal thoughts. (0 means inadequate information on the individual).
So the lower the number is, the higher likelihood of them not being able to function adequately and thus be seen as abnormal.