/ˈfoʊtɒn/
[noun] A tiny particle of light that has no mass and travels at the speed of light. Photons carry energy and are the basic units that make up all forms of light and other electromagnetic radiation.
The word 'photon' was invented in 1926 by a chemist — not a physicist — who used it to describe something Einstein had already discovered decades earlier.
At the heart of the word 'photon' lies the ancient Greek word phōs, meaning 'light', whose stem is phōt-. This root is one of the oldest and most poetic in the Greek language, appearing in mythology, philosophy, and early science alike. The Greeks used phōs to describe not just physical light but also the light of knowledge and understanding — a metaphor that has never really left us. From this root, Greek built the adjective phōteinos ('bright, luminous') and the verb phōtizein ('to illuminate'), and the stem passed into Latin and then into the scientific vocabulary of early modern Europe.
The modern word 'photon' was coined in 1926 by the American physical chemist Gilbert N. Lewis, who published a paper in the journal Nature proposing the term for what he described as 'an atom of light'. This is a curious historical irony, because Lewis's own theoretical model of the photon turned out to be largely incorrect — yet his name for the particle stuck permanently. The word was built simply and cleanly: Greek phōt- ('light') plus the suffix -on, which had already become a productive ending in physics for naming subatomic particles, following the discovery of the electron in 1897.
The suffix -on itself has a fascinating pedigree. It derives from the Greek neuter singular ending -on, and it entered particle physics through 'electron', a word coined in 1891 by the Irish physicist George Johnstone Stoney. Stoney built 'electron' from the Greek ēlektron, meaning 'amber' — because amber, when rubbed, produces static electricity. Once 'electron' became the template, the -on ending took on a life of its own, spawning proton (from Greek prōtos, 'first'), neutron (from Latin neuter, 'neither'), and dozens of other particle names throughout the 20th century. The photon joined this family in 1926 and became one of its most famous members.
Though Lewis named it, the physical concept behind the photon had been developing for over two decades. In 1900, the German physicist Max Planck proposed that light energy was emitted in discrete packets, which he called quanta (from Latin quantus, 'how much' or 'how great'). Then in 1905, Albert Einstein used this idea to explain the photoelectric effect — the way light can knock electrons off a metal surface — arguing that light itself travels as individual packets of energy, not just as a continuous wave. Einstein called these packets Lichtquanten, German for 'light quanta'. It was this concept that Lewis later named the 'photon', and it was this work that eventually won Einstein the Nobel Prize in Physics in 1921.
The Greek root phōt- has been extraordinarily productive in English, giving us a large family of familiar words. The path from ancient Greek phōs into English scientific vocabulary runs through New Latin and French translations used by 18th- and 19th-century scientists, who reached back to classical roots to name new discoveries. From phōt- we get 'photograph' (Greek phōt- + graphein, 'to write'), 'photography', 'photosynthesis' (Greek phōt- + synthesis, 'putting together'), 'photovoltaic', and 'phosphorus' — literally 'light-bearer' from Greek phōs + phoros ('carrying'). The word 'phone', however, is a false friend: it comes from the entirely different Greek root phōnē, meaning 'voice' or 'sound'. The photon, then, carries a name that links modern quantum physics to the oldest human desire to understand and name the light around us.