The S&P Completion Index is a sub-index of the S&P Total Market Index (S&P TMI), including all stocks eligible for the S&P TMI and excluding all current constituents of the S&P 500

S&P 500

The S&P 500 is a free-float capitalization-weighted index published since 1957 of the prices of 500 large-cap common stocks actively traded in the United States. The stocks included in the S&P 500 are those of large publicly held companies that trade on either of the two largest American stock...

. Covering approximately 4,000 constituents, the S&P Completion Index offers broad market exposure to mid-, small-, and micro-cap companies.

See http://www2.standardandpoors.com/spf/pdf/index/SP_Completion_Index_Factsheet.pdf for further information.