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Index
A number that sums up how a group of shares performs, such as the DAX or the S&P 500.
A stock index rolls many individual shares into one number. The DAX holds the 40 largest German listed companies, the S&P 500 about 500 large US companies, the FTSE 100 the 100 largest in London.
An index shows how "the market" as a whole is doing. That helps put moves in context: if a share falls 10 % while its index loses 8 %, the reason is more likely the overall market than the company. You cannot buy an index directly, but you can buy an ETF that tracks it.