= " the systematic application of economic theory, econometric techniques, and other formal or mathematical methods to the study of history". 
From the Wikipedia:
"The new economic history originated in 1958 with The Economics of Slavery in the Antebellum South by American economists Alfred H. Conrad and John R. Meyer. The book would cause a firestorm of controversy with its claim, based on statistical data, that slavery would not have ended in the absence of the U.S. Civil War, as the practice was economically efficient and highly profitable for slaveowners.
The term cliometrics—which derives from Clio, who was the muse of history—was originally coined by mathematical economist Stanley Reiter in 1960. Cliometrics became better known when Douglass North and William Parker became the editors of the Journal of Economic History in 1960. The Cliometrics Meetings also began to be held around this time at Purdue University and are still held annually in different locations.
North, a professor at Washington University in St. Louis, would go on to win the Nobel Memorial Prize in Economic Sciences in October 1993 along with Robert William Fogel, himself often described as the father of modern econometric history and Neo-historicals. The two were honoured "for having renewed research in economic history;" the Academy noted that "they were pioneers in the branch of economic history that has been called the 'new economic history,' or cliometrics." Fogel and North received the prize for turning the theoretical and statistical tools of modern economics on the historical past: on subjects ranging from slavery and railroads to ocean shipping and property rights. North was heralded as a pioneer in the "new" institutional history. In the Nobel announcement, specific mention was made of a 1968 paper on ocean shipping, in which North showed that organizational changes played a greater role in increasing productivity than did technological change. Fogel is especially noted for using careful empirical work to overturn conventional wisdom.
With that being said, the new economic history revolution is thought to have begun in the mid-1960s, where areas of key interest included transportation history, slavery, and agriculture. The discipline was resisted as many incumbent economic historians were either historians or economists who had very little connection to economic modeling or statistical techniques. According to cliometric economist Claudia Goldin, the success of the cliometric revolution had as an unintended consequence the disappearance of economic historians from history departments. As economic historians started using the same tools as economists, they started to seem more like other economists. In Goldin's words, "the new economic historians extinguished the other side." The other side nearly disappeared altogether, with only a few remaining in history departments and business schools. However, some new economic historians did, in fact, begin research around this time, among them were Kemmerer and Larry Neal (a student of Albert Fishlow, a leader of the cliometric revolution) from Illinois, Paul Uselding from Johns Hopkins, Jeremy Atack from Indiana, and Thomas Ulen from Stanford.
Cliometrics would be introduced to Germany by American-born and -educated Richard H. Tilly in the 1970s. The Cliometric Society, a group to encourage and further the study of cliometrics, was founded in 1983.
There has been a revival in 'new economic history' since the late 1990s. The number of papers on economic history published in the top economics journals has increased in the last decades, comprising 6.6% of articles in the American Economic Review and 10.8% of articles in the Quarterly Journal of Economics for the period 2004-2014. Today, cliometric approaches are standard in several journals, including the Journal of Economic History, Explorations in Economic History, the European Review of Economic History, and Cliometrica."