Question: What did Einstein suggest that radiation existed in?
About the article "Quantum", section "Etymology and discovery".

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In physics, the Einstein–Hopf drag (named after Albert Einstein and Ludwig Hopf) is a velocity-dependent drag force upon charged particles that are being bathed in thermal radiation. 
History of arguments on the properties of gravitational waves.
Einstein's double reversal.
The creator of the theory of general relativity, Albert Einstein, argued in 1916 that gravitational radiation should be produced, according to his theory, by any mass-energy configuration which has a time-varying quadrupole moment (or higher multipole moment).
Using a linearized field equation (appropriate for the study of "weak" gravitational fields), he derived the famous quadrupole radiation formula quantifying the rate at which such radiation should carry away energy.
Examples of systems with time varying quadrupole moments include vibrating strings, bars rotating about an axis perpendicular to the symmetry axis of the bar, and binary star systems, but not rotating disks.
In 1922, Arthur Stanley Eddington wrote a paper expressing (apparently for the first time) the view that gravitational waves are in essence ripples in coordinates, and have no physical meaning.
He did not appreciate Einstein's arguments that the waves are real.
In 1936, together with Nathan Rosen, Einstein rediscovered the Beck vacuums, a family of exact gravitational wave solutions with cylindrical symmetry (sometimes also called "Einstein-Rosen waves").
While investigating the motion of test particles in these solutions, Einstein and Rosen became convinced that gravitational waves were unstable to collapse.
Einstein reversed himself and declared that gravitational radiation was "not" after all a prediction of his theory.
Einstein wrote to his friend Max BornTogether with a young collaborator, I arrived at the interesting result that gravitational waves do not exist, though they had been assumed a certainty to the first approximation.
This shows that the nonlinear field equations can show us more, or rather limit us more, than we have believed up till now.
In other words, Einstein believed that he and Rosen had established that their new argument showed that the prediction of gravitational radiation was a "mathematical artifact" of the linear approximation he had employed in 1916.
Einstein believed these plane waves would gravitationally collapse into points; he had long hoped something like this would explain quantum mechanical wave-particle duality.
Einstein and Rosen accordingly submitted a paper entitled "Do gravitational waves exist?"
to a leading physics journal, the Physical Review, in which they described their wave solutions and concluded that the "radiation" which seemed to appear in general relativity was not genuine radiation capable of transporting energy or having (in principle) measurable physical effects.
The anonymous referee, who—as the current editor of the Physical Review recently confirmed, all parties now being deceased—was the combative cosmologist, Howard Percy Robertson, pointed out the error described below, and the manuscript was returned to the authors with a note from the editor asking them to revise the paper to address these concerns.
Quite uncharacteristically, Einstein took this criticism very badly, angrily replying "I see no reason to address the, in any case erroneous, opinion expressed by your referee."
He vowed never again to submit a paper to the Physical Review.
Instead, Einstein and Rosen resubmitted the paper without change to another and much less well known journal, the Journal of the Franklin Institute.
He kept his vow regarding the Physical Review.
Leopold Infeld, who arrived at Princeton University at this time, later remembered his utter astonishment on hearing of this development, since radiation is such an essential element for any classical field theory worthy of the name.
Infeld expressed his doubts to a leading expert on general relativity: H. P. Robertson, who had just returned from a visit to Caltech.
Going over the argument as Infeld remembered it (apparently from a conversation with Einstein), Robertson was able to show Infeld the mistake: locally, the Einstein-Rosen waves are gravitational plane waves (which had been studied earlier by O. R. Baldwin and George Barker Jeffery, and even earlier by Hans W. Brinkmann).
Einstein and Rosen had correctly shown that a cloud of test particles would, in sinusoidal plane waves, form caustics, but changing to another chart (essentially the Brinkmann coordinates) shows that the formation of the caustic is "not a contradiction at all", but in fact just what one would expect in this situation.
Infeld then approached Einstein, who concurred with this analysis (still not knowing it was he who reviewed the Physical Review submission).
Since Rosen had recently departed for the Soviet Union, Einstein acted alone in promptly and thoroughly revising their joint paper.
This third version was retitled "On gravitational waves", and, following Robertson's suggestion of a transformation to cylindrical coordinates, presented what are now called Einstein-Rosen cylindrical waves (these are locally isometric to plane waves).
This is the version which eventually appeared.
However, Rosen was unhappy with this revision and eventually published his own version, which retained the erroneous "disproof" of the prediction of gravitational radiation.
In a letter to the editor of the Physical Review, Robertson wryly reported that in the end, Einstein had fully accepted the objections which had initially so upset him. 
  Albert Einstein introduces the idea of stimulated radiation emission.  
In 1916, Albert Einstein proposed that there are three processes occurring in the formation of an atomic spectral line.
The three processes are referred to as spontaneous emission, stimulated emission, and absorption.
With each is associated an Einstein coefficient which is a measure of the probability of that particular process occurring.
Einstein considered the case of isotropic radiation of frequency , and spectral energy density . 
The following station of Einstein's scientific discoveries provides a context for the publications listed below, and clarifies the major themes running through his work.
The first four entries come from his Annus Mirabilis papers or miracle year papers.
In 1905, Einstein proposed the existence of the photon, an elementary particle associated with electromagnetic radiation (light), which was the foundation of quantum theory.
In 1909, Einstein showed that the photon carries momentum as well as energy and that electromagnetic radiation must have both particle-like and wave-like properties if Planck's law holds; this was a forerunner of the principle of wave–particle duality.
He would go on to receive the 1921 Nobel Prize in Physics for this work.
Likewise in 1905, Einstein developed a theory of Brownian motion in terms of fluctuations in the number of molecular collisions with an object, providing further evidence that matter was composed of atoms.
A few weeks earlier, he had derived the Einstein relation for diffusion, which was the first example of the general fluctuation-dissipation theorem and allowed a good estimate of the Avogadro constant.
Additionally In 1905, Einstein developed the theory of special relativity, which reconciled the relativity of motion with the observed constancy of the speed of light (a paradox of 19th-century physics).
Special relativity is now a core principle of physics.
Its counterintuitive predictions that moving clocks run more slowly, that moving objects are shortened in their direction of motion, and that the order of events is not absolute have been confirmed experimentally.
Also in 1905, Einstein developed his concept of Mass–energy equivalence.
Its relation "E=mc2" suggested that matter was a form of energy, which was later verified by the mass defect in atomic nuclei.
The energy released in nuclear reactions—which is essential for nuclear power and nuclear weapons—can be estimated from such mass defects.
In 1907 and again in 1911, Einstein developed the first quantum theory of specific heats by generalizing Planck's law.
His theory resolved a paradox of 19th-century physics that specific heats were often smaller than could be explained by any classical theory.
His work was also the first to show that Planck's quantum mechanical law "E=hν" was a fundamental law of physics, and not merely special to blackbody radiation.
Between 1907 and 1915, Einstein developed the theory of general relativity, a classical field theory of gravitation that provides the cornerstone for modern astrophysics and cosmology.
General relativity is based on the surprising idea that time and space dynamically interact with matter and energy, and has been checked experimentally in many ways, confirming its predictions of matter affecting the flow of time, frame dragging, black holes,</ref> and gravitational waves.
In 1917, Einstein published the idea for the Einstein–Brillouin–Keller method for finding the quantum mechanical version of a classical system.
The famous Bohr model of the hydrogen atom is a simple example, but the EBK method also gives accurate predictions for more complicated systems, such as the dinuclear cations H2+ and HeH2+.</ref>  In 1918, Einstein developed a general theory of the process by which atoms emit and absorb electromagnetic radiation (his "A" and "B" coefficients), which is the basis of lasers (stimulated emission) and shaped the development of modern quantum electrodynamics, the best-validated physical theory at present.
In 1924, together with Satyendra Nath Bose, Einstein developed the theory of Bose–Einstein statistics and Bose–Einstein condensates, which form the basis for superfluidity, superconductivity, and other phenomena.
In 1935, together with Boris Podolsky and Nathan Rosen, Einstein put forward what is now known as the EPR paradox, and argued that the quantum-mechanical wave function must be an incomplete description of the physical world.
In the final thirty years of his life, Einstein explored whether various classical unified field theories could account for both electromagnetism and gravitation and, possibly, quantum mechanics.
However, his efforts were unsuccessful, since those theories did not match experimental observations.