The FTSE 100 (Ticker:^FTSE) Index (pronounced footsie) is a share index of the 100 most highly capitalised companies listed on the London Stock Exchange, begun on 3 January 1984. The index was developed with a base level of 1000 on that date. Component companies must meet a number of requirements set out by the FTSE Group, including having a full listing on the London Stock Exchange with a Sterling or Euro dominated price on SETS, and meeting certain tests on nationality, free float, and liquidity.
The index is seen as a barometer of success of the British economy and is the leading share index in Europe. It is maintained by the FTSE Group, a now independent company which originated as a joint venture between the Financial Times and the London Stock Exchange. According to the FTSE Group’s website the FTSE 100 companies represent about 80% of the UK share market.
The constituents of the index are determined quarterly. (From Wikipedia)