Our definitions of homelessness are nearly as complicated as the issue themselves; it's only natural, considering how many different circumstances are all united under a single word. The Department of Housing and Urban Development's definition, simplified, describes a person as homeless when they lack a fixed, regular, and adequate nighttime residence. That is, they don't have a place to sleep at night that people should be sleeping in that they can stay regularly and that isn't likely to disappear on them.
Chronically homeless people have a mental and/or physical disability and have either been homeless for at least a year or have experienced at least four episodes of homelessness in the last three years.
When a homeless person is sheltered, they're living in an emergency shelter, in transitional housing, or in a safe haven. Thus, an unsheltered homeless person is living in places not meant for human habitation, like on the street or in abandoned buildings.
What information do we have?
The Annual Homeless Assessment Report to Congress is a publication created yearly by the Department of Housing and Urban Development. The AHAR presents the best statistics we have on the homeless population in the United States.
What is the U.S. doing about it?
In 2010, the Obama administration drafted and presented to Congress a federal plan to prevent and eliminate homelessness. That plan, updated in 2012 and 2015, is called "Opening Doors."