A term used by Nielsen Media Research to identify an exclusive geographic area of counties in which the home market television stations hold a dominance of total hours viewed. There are 210 DMA’s in the U.S. The DMA is composed of Sampling units (counties or geographically split counties) and is defined and updated annually by Nielsen Media Research, Inc. based on historical television viewing patterns. Nielsen reports radio listening estimates for the Top 50 DMAs (Ranked on TV Households) in the Radio Market Reports of all standard radio markets whose Metros are located within the DMA and whos names are contained in the DMA name.

Source:
http://www.nielsenmedia.com/glossary/
http://www.arbitron.com/downloads/terms_brochure.pdf