DMA (Designated Television Market) set up by the Nielsen Corporation, not the government.
It is a metropolitan area, including rural area surrounding it that is considered a TV market. In "theory" it's also about the same as the local TV stations coverage.
Everyone everywhere in the US, is in a DMA
They are rated by the number of households in each by a numbering system.
With NYC generally considered to be market #1. You will see other lists and even some variation in the Nielsen numbers depending on the year the statistic were grabbed. There are also other similar rating systems like READS on
RabbitEars.Info
I could go on and on but it's covered pretty well here
Whats DMA?
If you wonder what DMA you live in you can probably just query Google with the question for your city. If that doesn't work, tell us your city or list of stations you receive.