What is Quasi-steady regime (ARQS)?

A circuit of size $\ell$ driven at frequency $f$ is in the quasi-steady regime when the propagation time $\ell/c$ of electrical signals along it is negligible against the period $1/f$: $\ell \ll c/f$. The current is then the same at every point of an unbranched wire at each instant, and the laws below apply at each instant to time-varying currents and voltages (lowercase $i$, $u$) exactly as to steady ones ($I$, $U$).