The S&P 1500, or S&P Composite 1500 Index, is a of US stocks made by Standard & Poor's. It includes all stocks in the S&P 500, S&P 400, and S&P 600. This index covers 90% of the market capitalization of U.S. stocks. The index was launched on May 18, 1995.
There is an exchange-traded fund that tracks this index, iShares S&P 1500.
S&P also have the S&P 900 (S&P 500 plus S&P 400) index and the S&P 1000 (S&P 400 plus S&P 600) index.
The "S&P 1500" generally quoted is a price return index; there is also "total return" version of the index. These versions differ in how dividends are accounted for. The price return version does not account for dividends; it only captures the changes in the prices of the index components. The total return version reflects the effects of dividend reinvestment.