From SKYbrary Wiki
The N2 Indicator is a cockpit gauge which, in most installations, presents the rotational speed of the high pressure (high speed) engine spool, a speed that is referred to as N2. In a three spool engine, such as the Rolls Royce RB211, N2 is an indication of the intermediate pressure spool. The N2 Indicator is usually calibrated in percent RPM based on an engine manufacturer defined rotational speed that corresponds to 100%. The indicator can be a "stand alone" analogue or digital gauge or the information can be incorporated into the aircraft Engine Indicating and Crew Alerting System (EICAS) or Electronic Centralized Aircraft Monitor (ECAM) systems if installed.