What is Critical Mass?

Wikipedia gives this open and quite relevant definition of a Critical Mass:

Critical Mass is an event typically held on the last Friday of every month in cities around the world where bicyclists and, less frequently, unicyclists, skateboarders, inline skaters, roller skaters and other self-propelled commuters take to the streets en masse. While the ride was originally founded with the idea of drawing attention to how unfriendly the city was to bicyclists, the leaderless structure of Critical Mass makes it impossible to assign it any one specific goal. In fact, the purpose of Critical Mass is not formalized beyond the direct action of meeting at a set location and time and traveling as a group through city streets.

Most of all, Critical Mass is what its participants makes it. Some see it more of a street party, where cyclists gather together to celebrate the fact that they love bikes, whereas some see it more like an activit event, where somehing should be claimed (car-free streets, less pollution, etc.). A Critical Mass is on top of all a place where to meet people with the same passion as you: two wheels transportation.