The word quantum is the neuter singular of the Latin interrogative adjective quantus, meaning "how much". "Quanta", the neuter plural, short for "quanta of electricity" (electrons), was used in a 1902 article on the photoelectric effect by Philipp Lenard, who credited Hermann von Helmholtz for using the word in the area of electricity. However, the word quantum in general was well known before 1900,^[3] e.g. quantum was used in E. A. Poe's Loss of Breath. It was often used by physicians, such as in the term quantum satis, "the amount which is enough". Both Helmholtz and Julius von Mayer were physicians as well as physicists. Helmholtz used quantum with reference to heat in his article^[4] on Mayer's work, and the word quantum can be found in the formulation of the first law of thermodynamics by Mayer in his letter^[5] dated July 24, 1841.

In 1901, Max Planck used quanta to mean "quanta of matter and electricity",^[6] gas, and heat.^[7] In 1905, in response to Planck's work and the experimental work of Lenard (who explained his results by using the term quanta of electricity), Albert Einstein suggested that radiation existed in spatially localized packets which he called "quanta of light" ("Lichtquanta").^[8]

The concept of quantization of radiation was discovered in 1900 by Max Planck, who had been trying to understand the emission of radiation from heated objects, known as black-body radiation. By assuming that energy can be absorbed or released only in tiny, differential, discrete packets (which he called "bundles", or "energy elements"),^[9] Planck accounted for certain objects changing color when heated.^[10] On December 14, 1900, Planck reported his findings to the German Physical Society, and introduced the idea of quantization for the first time as a part of his research on black-body radiation.^[11] As a result of his experiments, Planck deduced the numerical value of h, known as the Planck constant, and reported more precise values for the unit of electrical charge and the Avogadro–Loschmidt number, the number of real molecules in a mole, to the German Physical Society. After his theory was validated, Planck was awarded the Nobel Prize in Physics for his discovery in 1918.

While quantization was first discovered in electromagnetic radiation, it describes a fundamental aspect of energy not just restricted to photons.^[12] In the attempt to bring theory into agreement with experiment, Max Planck postulated that electromagnetic energy is absorbed or emitted in discrete packets, or quanta.^[13]

• Aaronson Scott, Quantum computing since Democritus
• B. Hoffmann, The Strange Story of the Quantum, Pelican 1963. ^{[ISBN missing]}
• Lucretius, On the Nature of the Universe, transl. from the Latin by R.E. Latham, Penguin Books Limited., Harmondsworth 1951.
• J. Mehra and H. Rechenberg, The Historical Development of Quantum Theory, Vol.1, Part 1, Springer-Verlag, New York 1982. ^{[ISBN missing]}
• M. Planck, A Survey of Physical Theory, transl. by R. Jones and D.H. Williams, Methuen & Co., Limited., London 1925 (Dover editions 1960 and 1993) including the Nobel lecture. ^{[ISBN missing]}
• Rodney, Brooks (2011) Fields of Color: The theory that escaped Einstein. Allegra Print & Imaging. ^{[ISBN missing]}