The broadcast television signal operates by direct-wave radiation; the signal waves must travel in a straight line from the transmitting to the receiving antenna. Even if transmitters could operate with unlimited power, television broadcasting operates in a geography fixed by the horizon of the curve of the Earth’s surface. As the signal radiates outward from a transmitting antenna, it produces a more or less round geographical coverage pattern, with a radius of about 60 miles for VHF (very high frequency) stations and about 35 miles for UHF (ultrahigh frequency) stations. The coverage contour can be distorted by hills and mountains that block the signal, increased by antenna height, or added to by translators that rebroadcast the signal at another frequency in another location or by retransmission on cable television systems.

Reflecting the inherent “locatedness” of television broadcasting, the Federal Communications Commission (FCC) allocates channels and assigns licenses to facilities in communities. The word “market” has come to be the designator of those communities, reflecting the degree to which advertising dominates television in the United States. Anyone doing any type of business in an area may of course refer to that area or the people living in it as a market, placing the boundaries wherever sensible for the business in question. This practice includes the operators of commercial television. (The operators of noncommercial television facilities have less reason to use the word “market,” although it is increasingly applied in this arena.) In the business of television, these geographically outlined markets are formally defined by the ratings companies, among which Nielsen Media Research dominates.

Markets are defined by Nielsen as designated market areas (DMAs) in a manner essentially the same that the Arbitron company, which is no longer in the business of providing television ratings, once defined areas of dominant influence (ADIs). Both acronyms are still commonly used and designate essentially the same thing.

DMAs are defined by county or, in some cases, parts of counties (for convenience, counties will suffice in this discussion). Every county in the United States is assigned to one and only one DMA. Each DMA is named after the city that defines its center, such as the Chicago DMA or the Des Moines (Iowa) DMA. Each county is assigned to that DMA for which the most-watched television stations are broadcast. So, for example, Los Angeles County is assigned to the Los Angeles DMA because the television stations that the people in Los Angeles County watch most often are located in Los Angeles County. Orange County is also assigned to the Los Angeles DMA because the most frequently watched television stations by viewers in Orange County are also located in Los Angeles County.

Such a system of categories, in which every county in the United States is assigned to one and only one DMA, is considered mutually exclusive and exhaustive. Such systems have formal advantages. The key benefit here is the simple arithmetic for manipulating numbers associated with the categories. Since none of the markets overlap, numbers associated with any of them can be added together to describe a market that would be defined as the aggregate of the smaller markets. Since no area is left out of the system of market definitions, the sum of all of them defines the national market. This eases the calculation of ratings and other data for local, regional, or national markets and for syndicated, cable, and network television shows available in different areas.

In addition to these formal uses of the term “market,” as Nielsen’s DMA or regional or national aggregates of DMAs, there are various other uses for the term in the television business. One of the most common is in phrases such as “the African-American market,” “the Hispanic market,” “the youth market,” or “an upscale market.” These are extensions of the use of demographics to define types of people of interest to advertisers and other businesspeople. In either usage, the term remains a clear marker of the commercial aspects of the U.S. television industry, in which buying and selling—of both programs and audiences—is a central component.