Section 5150 is a section of the California Welfare and Institutions Code (specifically, the LantermanPetrisShort Act or "LPS") which allows a qualified officer or clinician to involuntarily confine a person suspected to have a mental disorder that makes him or her a danger to self, 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. When used as a term, 5150 (pronounced "fifty-one-fifty") can informally refer to the person being confined or to the declaration itself, or (colloquially) as a verb, as in "I have a possible 5150 here" or "(Someone) was 5150ed".
CALIFORNIA WELFARE AND INSTITUTIONS CODE, SECTION 5150, second paragraph, "... an application in writing stating the circumstances under which the person's condition was called to the attention of the officer, member of the attending staff, or professional person, and stating that the officer, member of the attending staff, or professional person has probable cause to believe that the person is, as a result of mental disorder, a danger to others, or to himself or herself, or gravely disabled."