Central business district, (CBD used in Australasia and sometimes elsewhere), or downtown (used mainly in the United States) are terms referring to the commercial centre of a city. The CBD or downtown is the central district of a city, usually characterized by a concentration of retail and commercial buildings. Although applicable to any city, both terms usually refer to larger cities.
The term center city is similar to CBD or downtown in that both serve the same purpose for the city, and both are recognizable by a higher-than-usual urban density as well as the tallest buildings in a city. Center city differs from downtown in that downtown can be geographically located anywhere in a city, while center city is located near the geographic center of the city. Examples of a center city can be found in Philadelphia, Houston, Pittsburgh, Baltimore, London, Sydney, and similar cities.