The definition of "urban" varies from country to country, and, with periodic reclassification, can also vary within one country over time, making direct comparisons difficult. An urban area can be defined by one or more of the following: administrative criteria or political boundaries (., area within the jurisdiction of a municipality or town committee), a threshold population size (where the minimum for an urban settlement is typically in the region of 2,000 people, although this varies globally between 200 and 50,000), population density, economic function (., where a significant majority of the population is not primarily engaged in agriculture, or where there is surplus employment) or the presence of urban characteristics (., paved streets, electric lighting, sewerage).
Carlos Slim topped the 2012 list, marking this third consecutive year at the top. Bill Gates placed second, but narrowed the gap from 2011 as Slim's fortune fell $5 billion while Gates' rose $5 billion. Warren Buffett remained in third place. Bernard Arnault of France was the top-ranking European on the list, placing fourth.  Ricardo Salinas Pliego was the greatest gainer in terms of dollars, adding $ billion to his fortune and moving up to number 37 overall.  Making her debut on the list at age 27, Spanx founder Sara Blakely became the youngest self-made female billionaire ever.  Colombia's Alejandro Santo Domingo was the highest-ranked newcomer, inheriting a $ billion stake in Santo Domingo Group from his father.  India's Lakshmi Mittal was the largest loser as his fortune dropped from $ billion to $ billion as the price of steel maker ArcelorMittal fell sharply.  As a result, he failed to make the top 10 for the first time since 2004 and lost his title of richest Asian to Hong Kong’s Li Ka-shing .