Definitions for "Cosmic rays"
Energetic atomic particles that are shot out of the Sun and strike the upper atmosphere of the earth generating muons which are detected by CCDs as a source of noise.
Atomic nuclei (mostly protons) the, are observed to strike the Earth's atmosphere with exceedingly high energies.
Ionizing radiation that originates from our sun and other stars in the universe. Cosmic rays are penetrating bundles of energy like gamma rays, but most are filtered out by the earth's atmosphere. The higher in altitude we go (e.g. mountains, jets), the more we can be exposed to them. Cosmic rays are part of background radiation.