The time that a party really "begins", no matter what scheduled time the party was actually supposed to start previous. This is the time that, without fail, droves of people show up all at once and a party can go from almost non-existent to full-fledged rager.
"Man, this party was so lame for like the past 3 hours, now it's insane! (Looking down at watch, seeing it's around 11 PM) 11 O'Clock Rule, happens every time!"
by Matty Rob April 06, 2009