Designated Market Area
In the television world, broadcasting areas are assigned by markets, or their Designated Market Area (DMA). This term was trademarked by Nielsen Media Research. In the United States, there are a total of 210 TV markets. A station's effective radiated power is controlled by the FCC to keep the signal localized to its market. This way, the signal cannot bleed into other markets (even though several do) and interfere with another market's television station; especially if these two stations carry similar affiliations.
Network affiliations and terrestrial syndication rights are assigned by market. It is not uncommon for a station in one market to carry more than one network affiliation.