What does the acronym LASER stand for at its first word?

The first word is Light, and the five letters expand to light amplification by stimulated emission of radiation. Each part describes the mechanism. Stimulated emission is the effect Einstein predicted in 1917: a photon passing an already excited atom can trigger it to drop energy level and release a second photon identical in wavelength, phase and direction. Amplification means arranging matters so this cascades, which requires a population inversion, more atoms excited than not, achieved by pumping energy in electrically or optically. The light bounces between mirrors at each end of the cavity, passing repeatedly through the medium and building up, with one mirror partly transparent to let the beam out. The result is why laser light behaves so differently from a torch: it is one wavelength, marching in step, and spreads very little with distance, which makes it useful for cutting, measuring, reading discs and carrying data down fibre.

The maser came first, doing the same thing with microwaves before anyone managed it with visible light.