A monthly gathering of people to ride bikes around a city or town. Its general purpose is to raise awareness that people ride bikes for transportation too and roads should be more accomidating for bikes. With many cyclists riding together in a group they are able to reach a "critical mass" and become a powerful entity of the roads, taking up many lanes.
Invented in San Francisco, it is celebrated in many cities across the world due to its simplicity to organize, often all you need to is spread the word about it. In San Francisco and New York participants often number in the thousands. It is usually at the end of the day on the last Friday of every month, at a standard, well-known gathering spot.
Often criticized by motorists as a nuciance, though cyclists criticise back that motorists hog the road, pollute, kill, etc.
Hey bro, want to see a movie this Friday at 5?
No man, I'm going to the Critical Mass to challenge the social norm of owning a car! Me and a few hundred other bike riders are going to be celebrating peaceful transportation. You should come. Everyone should come! No Critical mass? Start one!