Critical Mass
The notion that a sufficient number of adopters of an innovation in a social system is required such that the rate of adoption becomes self-sustaining and creates further growth.
Origin
Borrowed from nuclear physics — where it describes the minimum fissile material needed to sustain a chain reaction — the term was applied to social dynamics by scholars including Everett Rogers in his 1962 book Diffusion of Innovations. Thomas Schelling and Mark Granovetter developed threshold models in the 1970s explaining how individual decisions aggregate into collective tipping points, establishing the sociodynamic concept as a cornerstone of innovation and social change theory.
Everyday Use
A new social network isn't much fun with ten users — but once enough of your friends join, it becomes indispensable and attracts even more people. That tipping point is critical mass. You see it in movements, technologies, and trends: once enough people adopt something, it becomes self-reinforcing and nearly impossible to stop.