Each market represents a geographic area that is covered by a city's television stations. The New York City market is the nation's largest, with more than 7.5 million homes watching WABC, WCBS, WNBC, WNYW, WPIX, WWOR and the other local stations.
On the opposite end is tiny Glendive, Montana, with about 4,000 homes. It is the 210th TV market as defined by Nielsen.
Nielsen uses the term Designated Market Area (DMA) as its brand name for TV markets. You may also hear the term ADI, which stands for area of dominant influence, to describe TV markets.
It's important to remember that a TV market includes more territory than just the city where the TV stations broadcast. The New York City market represents homes in and around the city, plus northern New Jersey and small parts of Connecticut and Pennsylvania.
If households in a particular county watch New York City television stations more than 50% of the time, that county is considered part of the New York City market. If people in a county can also watch stations from another city and someday watch those stations more than half the time, Nielsen will switch the county into that other city's market.
TV market sizes grow or shrink based on population shifts. Nielsen publishes a list of its 210 DMAs each year, with markets often moving up or down a few places on the list.
A couple of the fastest-growing markets are Las Vegas, Nevada, and Ft. Myers-Naples, Florida. After Hurricane Katrina in 2005, the New Orleans, Louisiana, market suffered a large drop in market size as people left the city.