Market capitalization (often market cap) is a measurement of the size of a business corporation. It is equal to the price of one share of stock, times the number of shares of stock in a public company. Owning stock in a company is owning a part of the company. Market capitalization shows the public opinion of a company's value. The total market capitalization of all publicly traded companies in the world was US$51.2 trillion in January 2007. In May 2008 it rose to US$57.5 trillion, but by September 2008 had dropped to a little more than US$40 trillion. 