stimulated emission
An excited atom can drop to a lower level and emit a photon all by itself — spontaneous emission. But Einstein realized in 1917 that there must be a third process: if a photon of exactly the right energy is already passing by, it can trigger the excited atom to fall and emit a second photon. Crucially, that emitted photon is a perfect twin of the trigger — same frequency, direction, phase, and polarization. Stimulated emission is light making an identical copy of itself, and it is the physical seed of the laser.
Einstein's treatment balances three rates in equilibrium with radiation of spectral density rho(nu): absorption at rate B_12 rho N_1, spontaneous emission at rate A_21 N_2, and stimulated emission at rate B_21 rho N_2, where N_1 and N_2 are the lower and upper populations. Demanding consistency with the Planck spectrum and Boltzmann populations forces the Einstein relations B_12 = B_21 (for equal degeneracies) and A_21/B_21 = 8 pi h nu^3/c^3. Because stimulated and absorbed photons are governed by the same coefficient, a beam is amplified only when there are more atoms up than down: net gain requires N_2 > N_1, a population inversion.
Stimulated emission is why coherent light amplification is possible at all and, run in reverse, why a medium normally absorbs. The unavoidable caveat is that population inversion is impossible in thermal equilibrium: the Boltzmann factor guarantees N_2 < N_1 at any positive temperature, so a laser must be actively pumped into a non-equilibrium state to invert its populations. And stimulated emission must not be confused with spontaneous emission, which shares none of the trigger photon's phase or direction and is the source of an atom's random, incoherent glow.
The Einstein relation A_21/B_21 = 8 pi h nu^3/c^3 shows spontaneous emission dominates at high frequency. At optical frequencies spontaneous emission easily outcompetes stimulated emission in ordinary light — which is why a light bulb is incoherent, and why making a laser requires both an inversion and a cavity to build up the stimulating field.
The nu^3 factor is why coherent light was hard to make until pumping and cavities arrived.
Stimulated emission by itself does not make a laser — without a population inversion the same coupling makes the medium a net absorber, so pumping to a non-equilibrium inversion is the real prerequisite.