1. A dedicated device designed for the purpose of monitoring network traffic in order to recognize and decode certain packets of interest. 2. A software package that enables a general-purpose computer to recognize and decode certain packets of interest. Note: The packet sniffer is normally used by system administrators for network management and diagnostics, but is occasionally used by hackers for illicit purposes such as stealing a user's password or credit-card number. 3. In INFOSEC, synonym yysniffer.