What does SIP mean for an electronic stock exchange?

Securities Information Processor. They're the ones that implement "the tape". For example, IBM trades on multiple exchanges/ECNs. Each exchange/ECN reports bid/ask and prints trades to a common reporting facility - the SIP. For NYSE,AMEX listed securites, the SIP is the CTA (Consolidated Tape Association) and they disseminate this information real-time through the market data feeds CTS (Consolidated Tape System) and CQS (Consolidated Quote Syetem). NASDAQ-listed securities have their own SIP and tape system, as do the options market.