From the U.S. Bureau of Labor Statistics document "Labor Market Areas, 2008" (from http://www.bls.gov/lau/lmadir.pdf; also defined within http://www.bls.gov/lau/laugeo.htm):
A general definition for a labor market area is an economically integrated geographic area within which individuals can reside and find employment within a reasonable distance or can readily change employment without changing their place of residence. Labor market areas are metropolitan areas, micropolitan areas, or small labor market areas. They exhaust the geography of all States, the District of Columbia, and Puerto Rico, with the exceptions of Kalawao County, Hawaii, and 18 isolated minor civil divisions (MCDs) in New England. Labor market area definitions are updated on an annual basis, and changes to area definitions and titles are introduced with the labor force estimates for the following January. In order to maintain a consistent time series, data for labor market areas generally are reconstructed back to January 1990 or as far back as practicable.
A general definition for a labor market area is an economically integrated geographic area within which individuals can reside and find employment within a reasonable distance or can readily change employment without changing their place of residence.
Labor market areas are metropolitan areas, micropolitan areas, or small labor market areas. They exhaust the geography of all States, the District of Columbia, and Puerto Rico, with the exceptions of Kalawao County, Hawaii, and 18 isolated minor civil divisions (MCDs) in New England.
Labor market area definitions are updated on an annual basis, and changes to area definitions and titles are introduced with the labor force estimates for the following January. In order to maintain a consistent time series, data for labor market areas generally are reconstructed back to January 1990 or as far back as practicable.
The BLS document is attached to this article.