The solar wind is a stream of charged particles released from the Sun's outermost atmospheric layer, the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between 0.5 and 10 keV. The composition of the solar wind plasma also includes a mixture of particle species found in the solar plasma: trace amounts of heavy ions and atomic nuclei of elements such as carbon, nitrogen, oxygen, neon, magnesium, silicon, sulfur, and iron. There are also rarer traces of some other nuclei and isotopes such as phosphorus, titanium, chromium, and nickel's isotopes ⁵⁸Ni, ⁶⁰Ni, and ⁶²Ni. Superimposed with the solar-wind plasma is the interplanetary magnetic field. The solar wind varies in density, temperature and speed over time and over solar latitude and longitude. Its particles can escape the Sun's gravity because of their high energy resulting from the high temperature of the corona, which in turn is a result of the coronal magnetic field. The boundary separating the corona from the solar wind is called the Alfvén surface.

At a distance of more than a few solar radii from the Sun, the solar wind reaches speeds of 250–750 km/s and is supersonic, meaning it moves faster than the speed of fast magnetosonic waves. The flow of the solar wind is no longer supersonic at the termination shock. Other related phenomena include the aurora (northern and southern lights), comet tails that always point away from the Sun, and geomagnetic storms that can change the direction of magnetic field lines.

The existence of particles flowing outward from the Sun to the Earth was first suggested by British astronomer Richard C. Carrington. In 1859, Carrington and Richard Hodgson independently made the first observations of what would later be called a solar flare. This is a sudden, localised increase in brightness on the solar disc, which is now known to often occur in conjunction with an episodic ejection of material and magnetic flux from the Sun's atmosphere, known as a coronal mass ejection. The following day, a powerful geomagnetic storm was observed, and Carrington suspected that there might be a connection; the geomagnetic storm is now attributed to the arrival of the coronal mass ejection in near-Earth space and its subsequent interaction with the Earth's magnetosphere. Irish academic George FitzGerald later suggested that matter was being regularly accelerated away from the Sun, reaching the Earth after several days.

In 1910, British astrophysicist Arthur Eddington essentially suggested the existence of the solar wind, without naming it, in a footnote to an article on Comet Morehouse. Eddington's proposition was never fully embraced, even though he had also made a similar suggestion at a Royal Institution address the previous year, in which he had postulated that the ejected material consisted of electrons, whereas in his study of Comet Morehouse he had supposed them to be ions.

The idea that the ejected material consisted of both ions and electrons was first suggested by Norwegian scientist Kristian Birkeland. His geomagnetic surveys showed that auroral activity was almost uninterrupted. As these displays and other geomagnetic activity were being produced by particles from the Sun, he concluded that the Earth was being continually bombarded by "rays of electric corpuscles emitted by the Sun". He proposed in 1916 that, "From a physical point of view it is most probable that solar rays are neither exclusively negative nor positive rays, but of both kinds"; in other words, the solar wind consists of both negative electrons and positive ions. Three years later, in 1919, British physicist Frederick Lindemann also suggested that the Sun ejects particles of both polarities: protons as well as electrons.

Around the 1930s, scientists had concluded that the temperature of the solar corona must be a million degrees Celsius because of the way it extended into space (as seen during a total solar eclipse). Later spectroscopic work confirmed this extraordinary temperature to be the case. In the mid-1950s, British mathematician Sydney Chapman calculated the properties of a gas at such a temperature and determined that the corona being such a superb conductor of heat, it must extend way out into space, beyond the orbit of Earth. Also in the 1950s, German astronomer Ludwig Biermann became interested in the fact that the tail of a comet always points away from the Sun, regardless of the direction in which the comet is travelling. Biermann postulated that this happens because the Sun emits a steady stream of particles that pushes the comet's tail away. German astronomer Paul Ahnert is credited (by Wilfried Schröder) as being the first to relate solar wind to the direction of a comet's tail based on observations of the comet Whipple–Fedke (1942g).

American astrophysicist Eugene Parker realised that heat flowing from the Sun in Chapman's model, and the comet tail blowing away from the Sun in Biermann's hypothesis, had to be the result of the same phenomenon which he termed the "solar wind". In 1957, Parker showed that although the Sun's corona is strongly attracted by solar gravity, it is such a good conductor of heat that it is still very hot at large distances from the Sun. As solar gravity weakens with increasing distance from the Sun, the outer coronal atmosphere is able to escape supersonically into interstellar space. Parker was also the first person to notice that the weakening influence of the Sun's gravity has the same effect on hydrodynamic flow as a de Laval nozzle, inciting a transition from subsonic to supersonic flow. There was strong opposition to Parker's hypothesis on the solar wind; the paper he submitted to The Astrophysical Journal in 1958 was rejected by two reviewers, before being accepted by the editor Subrahmanyan Chandrasekhar.

In January 1959, the Soviet spacecraft Luna 1 first directly observed the solar wind and measured its strength, using hemispherical ion traps. The discovery, made by Konstantin Gringauz [ru], was verified by Luna 2, Luna 3, and the more distant measurements of Venera 1. Three years later, a similar measurement was performed by American geophysicist Marcia Neugebauer and collaborators using the Mariner 2 spacecraft.