Origins
The word 'laser' is one of the most successful acronyms in the history of the English language, so thoroughly absorbed into everyday vocabulary that most speakers are unaware it stands for 'Light Amplification by Stimulated Emission of Radiation.' It is a product of the mid-twentieth-century American physics community, coined during one of the most consequential races in the history of technology.
The acronym was modeled on the earlier 'maser' (Microwave Amplification by Stimulated Emission of Radiation), a term coined in 1955 by Charles H. Townes and Arthur Schawlow to describe a device that produced coherent microwave radiation. When physicists began working on extending the principle to visible light, the obvious naming strategy was to replace 'Microwave' with 'Light,' yielding 'laser.'
The physicist Gordon Gould is generally credited with coining the term. In November 1957, Gould wrote 'LASER' in his laboratory notebook at Columbia University, defining it as 'Light Amplification by Stimulated Emission of Radiation.' However, Theodore Maiman built the first functioning laser — a ruby laser — at Hughes Research Laboratories in May 1960, and the word entered widespread use from that point. The priority dispute between Gould, Townes, and Schawlow over both the word and the patent rights became one of the most famous in the history of science.
Development
The theoretical foundation for the laser was laid by Albert Einstein in 1917, when he described the process of stimulated emission in his paper 'Zur Quantentheorie der Strahlung' (On the Quantum Theory of Radiation). Einstein showed that an incoming photon could stimulate an excited atom to emit a second photon identical in frequency, phase, and direction — the principle that makes laser light coherent.
Each word in the acronym has its own etymological depth. 'Light' comes from Old English 'lēoht,' from Proto-Germanic '*leuhtą,' from PIE *lewk- (light, brightness) — the same root that produced Latin 'lūx' (light), 'lūcidus' (clear, bright), and 'lūmen' (light). 'Amplification' comes from Latin 'amplificāre' (to make larger), from 'amplus' (large, spacious). 'Stimulated' comes from Latin 'stimulāre' (to goad, to urge), from 'stimulus' (a pointed stick for driving cattle). 'Emission' comes from Latin 'ēmittere' (to send out). 'Radiation' comes from Latin 'radiātiō' (a shining), from 'radius' (ray, beam, spoke of a wheel).
The word 'laser' has undergone complete lexicalization — the process by which an acronym loses its uppercase spelling and is treated as an ordinary word. This is evident in several ways: it is universally written in lowercase; it takes standard English inflections ('lasers,' 'lasered'); and it has produced a back-formation verb, 'to lase' (to emit coherent laser light), which was reverse-engineered from the noun as if 'laser' were an agent noun (one who lases). The existence of 'lase' demonstrates that speakers parse 'laser' as a root word, not as an acronym.