Definition
Critical mass is the point at which a process accumulates enough of something — material, participants, energy, momentum — to become self-sustaining. Below the threshold, the process fizzles; above it, the process feeds itself and runs without further outside push.
The phrase comes from nuclear physics, where a quantity of fissile material below a certain mass cannot sustain a chain reaction, while a quantity above it can. The idea generalises cleanly: a social movement that has too few members loses energy faster than it gains it, while one past its threshold recruits faster than it loses, and a marketplace with too few buyers and sellers stalls until each side becomes worth showing up for.
Why it matters
How it works
A self-sustaining process has two competing flows: a gain that grows the system and a loss that drains it. Below critical mass, loss outpaces gain and the system decays toward zero. Above it, gain outpaces loss and the system grows on its own. The threshold is simply where the two rates cross.
This makes the concept a planning tool. Instead of asking whether a project is progressing, ask whether it has crossed the line beyond which it no longer needs external energy. If not, the realistic options are to add enough push to clear the threshold or to stop — sustained effort that never reaches critical mass yields nothing lasting.