ADS. Computerized detection system utilized by the National Association of Securities Dealers' (NASD) that automatically monitors trades and execution orders made by firms to prevent irregularities that may be prohibited by regulations. The system has been in existence since 1997 and monitors over 2 million transactions made in the Nadaq stock market. The system is able to monitor a large portion of the market that previously could not be monitored on a consistent basis. also called Advance Computerized Execution System (ACES).