The term "Cosmic Rays" refers to elementary particles, nuclei, and electro-magnetic radiation of extra-terrestrial origin. These may include exotic, short-lived particles such as muons, pi-mesons or lambda baryons.
In the energy range of 1012-1015 eV (electron-volts*), cosmic rays arriving at the edge of the Earth's atmosphere have been measured to consist of approximately:
*An electron-volt is a microscopic unit corresponding to the energy gained or lost by an electron in crossing a potential gap of 1 V(volt). A typical battery (e.g. a AA-cell used in remote control units for television sets) has a voltage difference of 1.5 V. For comparison: a 40 W(watt) reading-light uses about 1024 eV of energy in one hour.
At the start of the 1900's, French physicist Henri Becquerel discovered that certain elements are unstable, and would transmute into other elements, and in the process, emit what appeared to be particles. These "particles" were given the name "radiation", and the process itself referred to as "radioactive decay".
It was noticed that an instrument called an "electroscope" would spontaneously discharge in the presence of radioactive materials. The rate of discharge of an electroscope is then used as a measure of the level of radiation. The electroscope thus became a standard instrument for studying radiation and radioactive materials in the first decades of the Twentieth Century.
However, physicists noticed that electroscopes were found to discharge slowly even in the absence of radioactive matter. This residual discharge could not be attributed to leakage. There appears to be a background radiation.
To study the source of this background, Austrian physicist Victor. F. Hess made measurements of radiation levels at different altitudes with electroscopes aboard a balloon. The motivation for this study was to distance the electroscopes from radiation sources in the Earth. Hess went as high as 17,500 feet in his balloon without oxygen tanks. Surprisingly, he found that the radiation levels increased with altitude. Hess interpreted this result to mean that radiation is entering the atmosphere from outer space. He gave this phenomenon the name "Cosmic Radiation", which later evolved to "Cosmic Rays". Hess was awarded the Nobel Prize in 1936 for his discovery of cosmic rays.