Frederick is a city in, and the county seat of, Frederick County, Maryland, United States. Frederick's population was 78,171 people as of the 2020 census, making it the second-largest incorporated city in Maryland behind Baltimore. It is a part of the Washington metropolitan area and the greater Washington–Baltimore combined statistical area. The city is located at an important crossroads at the intersection of a major north–south Native American trail and east–west routes to the Chesapeake Bay, both at Baltimore and what became Washington, D.C., and across the Appalachian mountains to the Ohio River watershed. Frederick is home to Frederick Municipal Airport (IATA: FDK), which accommodates general aviation, and Fort Detrick, a U.S. Army bioscience and communications research installation and Frederick County's largest employer.