Jefferson County (JeffCo) , whose slogan is the "Gateway to the Rocky Mountains", is the fourth most populous of the 64 counties of the State of Colorado of the United States. Located along the Front Range of the Rocky Mountains, Jefferson County is adjacent to the west side of the state capital, Denver. The county population was 534,543 according to the 2010 census. The county seat is Golden and the most populous city is Lakewood. Jefferson County is part of the Denver-Aurora Metropolitan Statistical Area and the Denver-Aurora-Boulder Combined Statistical Area. As of 2002, Jefferson County contains the state's center of population.
Jefferson County is commonly nicknamed Jeffco. The name Jeffco is incorporated in the name of the Jeffco School District, the Jeffco Business Center Metropolitan District No. 1, and several businesses located in Jefferson County. Jeffco is also incorporated in the unofficial monikers of many Jefferson County agencies. The Rocky Mountain Metropolitan Airport operated by Jefferson County was previously known as the Jeffco Airport.
A major employer in Jefferson County is the large Coors Brewing Company in Golden. Also, the state-supported Colorado School of Mines is located in Jefferson County, offering programs in STEM topics such as mining, geology, chemistry, and engineering.