A simulation of a chemical process might involve a substance f increasing during a process, and a substance g decreasing during the same process. The process should be terminated once both substances have reached the same level. Both substance concentrations are time dependent, so f = f(t) and g = g(t). Equilibrium is reached at the time t when f(t) = g(t). Setting

y(t) = f(t)− g(t),

we are now looking for the root of y(t). The term “root” refers to the zero-crossing of the function y, that is, to finding the value t for which

y(t) = 0.