The term business cluster, also known as an industry cluster, competitive cluster, or Porterian cluster, was introduced and popularized by Michael Porter in "The Competitive Advantage of Nations" (1990). The importance of economic geography, or more correctly geographical economics, was also brought to attention by Paul Krugman in "Geography and Trade" (1991). Cluster development has since become a focus for many government programs. The underlying concept, which economists have referred to as agglomeration economies, dates back to 1890, and the work of Alfred Marshall.