Cosmic ray
WordNet
noun
(1) 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
WiktionaryText
Noun
- An energetic particle originating outside our solar system.
- Cosmic radiation: a stream of cosmic rays.
Usage notes
The use of the words "ray" and "radiation" implies incorrectly that cosmic rays are electromagnetic waves, similar to light rays and X-rays; cosmic rays were so named before their nature was known.