cosmic ray

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A very rapidly moving particle of extraterrestrial origin, especially one that collides with the earth's atmosphere. Cosmic rays are mainly protons and alpha particles but sometimes are other light atomic nuclei and high-energy electrons.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An energetic particle originating outside our solar system.
• noun Cosmic radiation: a stream of cosmic rays.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun highly penetrating ionizing radiation of extraterrestrial origin; consisting chiefly of protons and alpha particles; collision with atmospheric particles results in rays and particles of many kinds