Section 5150 is a section of the California Welfare and Institutions Code (WIC) (in particular, the Lanterman–Petris–Short Act or "LPS") which authorizes a qualified officer or clinician to involuntarily confine a person suspected to have a mental disorder that makes them a danger to themselves, a danger to others, and/or gravely disabled. A qualified officer, which includes any California peace officer, as well as any specifically-designated county clinician, can request the confinement after signing a written declaration stating the psychiatric diagnosis that the diagnosing medical professional believes to be the cause or reason why they believe the patient to be "a danger to themselves or others" or the psychiatric disorder that has rendered the patient incapable of making their own medical treatment decisions.
In informal usage, 5150 (pronounced "fifty-one-fifty") can refer to the person being confined (e.g., "I have a possible 5150 here"), the declaration, or the act of committing someone (as in "(Someone) was 5150ed"). Also it is used as police slang for somebody that is crazy, or committed a stupid act. "The guy must have been 5150 to drive down the wrong side of the road."
Largely because the production of many American movies and television programs are based in California, usage of the term "5150" has spread beyond its original location and user population. An album of the same name by the California hard rock band Van Halen was named directly for the code section, and derivative uses followed.