Helen is correct; the definition of a city is controlled by the states.
In Maryland, as in NY, a city is an incorporated area. If I mistake not, the only cities in Maryland are Baltimore and Cumberland.
This makes for some strange situations. One of the newest and largest urban areas is Columbia, a planned city which is not a city; in fact, it's not even a town. Columbia is located at the junction of 4 different counties and is, legally, merely a collection of housing tracts located in each of the 4 counties. In many parts of it, people on one side of the street pay different taxes, have different representatives in the state legislature and U.S. Congress, and have different police and fire depts. than their neighbors on the other side of the street.
Then there is Baltimore (the City of), which is not located in any county; it's a separate subdivision of the state all by itself. There is a Baltimore County, but the City of Baltimore is not part of it. It has its own governmental setup, taxes, etc. which counties have.
