In the Diagnostic and Statistical Manual of Mental Disorders (DSM), the official guidebook for diagnosing psychological disorders published by the American Psychiatric Association, diagnoses can be made on five different levels. Each level is called an "axis."
On the first level, or axis, major mental disorders that are considered to be "clinical disorders" are diagnosed. These are the disorders that most people think of when they think of psychological conditions (e.g., major depressive disorder, schizophrenia, bipolar disorder).
On the second axis, disorders that are long-standing are diagnosed. This is the level where personality disorders are diagnosed, and it is the reason they are often called "Axis II" disorders. This system of separating out personality disorders on to a separate axis was developed so that the person doing the assessment would not pay so much attention to the sometimes more obvious Axis I conditions that they overlook the personality disorders or other long-standing conditions.
The last three levels are used to diagnose general medical conditions that impact mental health, social or environmental problems the person is experiencing, and overall functioning.
There is considerable debate among experts about whether personality disorders really belong on a separate axis, or whether the system needs to be revised. It is possible that when the next version of the DSM is published, personality disorders will no longer reside on Axis II.
American Psychiatric Association. Diagnostic and statistical manual of mental disorders, 4th ed, text revision. Washington, DC, Author, 2000.
Bernstein, DP, Iscan, C, Maser, J, Board of Directors, Association for Research in Personality Disorder, & Board of Directors, International Society for the Study of Personality Disorders. "Opinions of personality disorder experts regarding the DSM-IV Personality Disorders classification system." Journal of Personality Disorders, 21:536-551, 2007.