FTSE 100 Index

The FTSE 100 (often said 'foot-sie') is an important stock exchange in London, United Kingdom, similar to the Dow Jones Industrial Average, in the United States, or the CAC 40, in France. FTSE stands for Financial Times Stock Exchange. The 100 biggest companies in the United Kingdom are listed on it. The index took the place of the FT30 in January 1984. 
It is calculated by computers in real time, and the cost of shares in the companies is reported every 15 seconds.