equilibrium

A state where opposing influences cancel exactly, so the system stops changing. _Mechanical_ equilibrium: net force is zero, so acceleration is zero (Newton's first law). _Chemical_ equilibrium: forward and reverse reaction rates match, so concentrations are stable. _Economic_ equilibrium: supply and demand price match, so the market price stops drifting. The shared shape across all of them: a quantity has two competing influences that depend on the quantity itself, and the _fixed point_ where they balance is the equilibrium. Terminal velocity is the equilibrium of `dv/dt = g − kv` — the velocity at which "gravity adds to v" and "drag removes from v" cancel.