California Public Policy and Citizen Participation/Homelessness
Homelessness describes the condition of people without a regular dwelling. People who are homeless are unable or unwilling to acquire and maintain regular, safe, and adequate housing, or lack "fixed, regular, and adequate night-time residence." The legal definition of "homeless" varies from country to country, or among different entities or institutions in the same country or region.
The term homeless may also include people whose primary night-time residence is in a homeless shelter, a warming center, a domestic violence shelter or other ad hoc housing situation. Government homeless enumeration studies  also include persons who sleep in a public or private place not designed for use as a regular sleeping accommodation for human beings.
Difficulties in classification: lack of generally agreed nomenclature
The "unsheltered" are that segment of the homeless who do not have ordinary lawful access to buildings in which to sleep. Such persons frequently prefer the term "houseless" to the term "homeless". Others may use the term street people which does not fully encompass all unsheltered in that many such persons do not spend their time on urban "street" environments, and to the contrary shun such locales and prefer to convert unoccupied buildings, or to inhabit mountains or, more often, lowland meadows, creeks and beaches
A portion of homeless persons are generally in transit, but there is no generally accepted terminology to describe them; some nomenclature is frequently associated with derogatory connotations, and thus the professional and vernacular lingo to describe these persons is both evolving and not lacking in controversy. Much of the concern stems from the European situation, where "homeless" persons of Romani people|Roma, Sinti and other ethnic descent have rejected the term "gypsy". Other terms which some use regarding in-transit persons are: transient, vagabond, tramp or "railroad tramp". Occasionally these terms are interchanged with terms not necessarily implying that the person is a traveler, i.e. hobo. The term "bum" is used for persons lacking a work ethic, or ethics generally, but its application to persons purely on the basis of homelessness is strictly a form of prejudicial labeling. The term "transient" is frequently used in police reports, without any precise definitions across jurisdictions.
Many jurisdictions have developed programs to locate such persons in short term emergency shelter, often in churches or other institutional real property, during particularly cold spells. These are referred to as warming centers, and are credited by their advocates as life saving. 
United Nations definition
The UN, either via a UN Agency or via a vote in the General Assembly has agreed upon these bare minimum conditions for a person to be countable as homeless :
- The person is assumed not to own any other property that is habitable in or on
- Their rent or mortgage are equal to or greater than 72% of their monthly income
- Their source of income to pay rent is not stable or the amount of money available for food is not stable
It is understood that these legal definitions for homelessness may date back to the beginnings of the UN in the late 1940s. The definition of a person being a refugee is at least partly linked to the definition of homelessness as many refugees may have been or are homeless. Trends in homelessness are closely tied to neighborhood conditions. 
HUD technical definitions of homelessness
Homelessness in the United States is addressed by United States Department of Housing and Urban Development which promulgates best practices and definitions which are highly influential. HUD defines homeless as pertinent to an individual who lacks
- an individual who lacks a fixed, regular, and adequate night-time residence; and
- an individual who has a primary night-time residence that is -
- a supervised publicly or privately operated shelter designed to provide temporary living accommodations (including welfare hotels, congregate shelters, and transitional housing for the mentally ill);
- an institution that provides a temporary residence for individuals intended to be institutionalized; or
- a public or private place not designed for, or ordinarily used as, a regular sleeping accommodation for human beings
Chronic homelessness, an area of emerging concern
(HUD) defines a "chronically homeless" person as "an unaccompanied homeless individual with a disabling condition who has either been continuously homeless for a year or more, or has had at least four episodes of homelessness in the past three years." Much of the current literature takes note that such persons often account for a disproportionately large percent of the public and private expenditures related to homelessness. In response to this perception, there is as of early 2011 an increased momentum, particularly on both US coasts, to "target" the chronically homeless utilizing a vulnerability index system. These "VI's" code for conditions such as advance kidney or liver disease, HIV-AIDS or multiple hospitalization as well as factors such as age. Criticism has been relatively muted as parties often critical of conventional homeless service systems recognize a window of opportunity for innovations. Common Ground is an effort which originated in NYC, claiming high success in the Times Square area, and now proliferating its model in other areas such as Los Angeles. 
Area of emerging interest: the unsheltered
The "unhoused" are that segment of the homeless who do not have ordinary lawful access to buildings in which to sleep, as referred to in the HUD definition as persons occupying "place not designed for ... sleeping accommodation for human beings. Such persons frequently prefer the term "houseless" to the term "homeless". Recent homeless enumeration survey documentation utilizes the term "unsheltered homeless." HUD requires jurisdictions which participate in Continuum of Care grant programs to count their homeless every two years. These counts have led to a variety of creative measures to avoid undercounting. Thus teams of counters, often numbering in the hundreds in logistically complex volunteer efforts, seek out the unsheltered in various nooks and crannies. There has been a significant number of unsheltered persons dying of hypothermia, adding impetus to the trend of establishing warming centers as well as extending the enumeration surveys with vulnerability indexes 
- United States Department of Housing and Urban Development, "Federal Definition of Homeless"
- "Glossary defining homelessness"
- Bogard, Cynthia J., "Advocacy and Enumeration: Counting Homeless People in a Suburban Community", American Behavioral Scientist September 2001 vol. 45 no. 1 105-120
- Gabbard, W. Jay; et al, "Methodological Issues in Enumerating Homeless Individuals", Journal of Social Distress and the Homeless Volume 16, Number 2 / May 2007 90-103
- Office of Applied Studies, United States Department of Health and Human Services,"Terminology"
- United States Code, Title 42, Chapter 119, Subchapter I, § 11302. United States Code: General definition of a homeless individual.
- "Inside Straight Edge". Writer: David Shadrack Smith. Directors: Jim Gaffey and David Shadrack Smith. Inside. National Geographic Society. 9 April 2008. Retrieved on 28 January 2011.
- Svitek, Patrick. "Evanston homeless find warm shelters". Daily Northwestern. http://www.dailynorthwestern.com/city/evanston-homeless-find-warm-shelters-1.2458986. Retrieved
- Families on the move: Breaking the cycle of homelessness.,Notkin, S., Rosenthal, B., & Hopper, K., New York: Edna McConnell Clark Foundation Ken Burnett, 1990
- United States Department of Housing and Urban Development (HUD), "Federal Definition of Homeless"
- HUD working definition of Chronically homeless
- Common Ground website New York City
- "Detroit cuts funding for homeless warming centers". Wsws.org. 2011-01-29. http://www.wsws.org/articles/2011/jan2011/home-j29.shtml. Retrieved 2011-02-
- Hendricks, Mike (2011). "After judge’s warning, homeless camp with TV, Christmas lights is taken apart". The Kansas City Star. http://www.kansascity.com/2011/01/18/2593523/after-judges-warning-homeless.html#. Retrieved 14 February 2011.