Definition - What does Radiation mean?
Radiation is energy that emits from a source in the form of electromagnetic waves or subatomic particles. Light, sound and heat are all types of radiation. The radiation that is associated with radioactive materials is a subatomic particle which can ionize the particles in matter or produce particles in matter that have a charge. This is caused by unstable atoms, which are said to be radioactive.
Safeopedia explains Radiation
Radioactive or unstable atoms have an excess of mass or energy, or both. As an atom regains it's stability it gives off excess energy or mass in the form of electromagnetic waves or subatomic particles. These subatomic particles can cause ionization in surrounding material and this can be hazardous to human health. Light is an example of electromagnetic radiation, as well as X-rays or gamma rays. Alpha and Beta radiation are examples of particulate radiation.