The S&P/ASX 300, or simply, ASX 300, is a stock market index of Australian stocks listed on the Australian Securities Exchange. The index is market-capitalisation weighted, meaning each company included is in proportion to the index's total market value, and float-adjusted, meaning the index only considers shares available to public investors.
The index measures the performance of the top 300 companies listed on the ASX. The index was formed in April 2000, by Standard and Poor's Dow Jones Indices. It was created to provide broader exposure to the Australian equity market compared to the S&P/ASX 200. The index incorporates all the companies within the S&P/ASX 200 and adds 100 more companies based on their market capitalisation. Index components are reviewed semi-annually by Standard & Poor's. The average annual total return of the index is 19.3% as of 08/04/2020, however, there have been multiple periods where the index fell over 30%.Wikipedia