Radiation is simply energy interacting with matter. Energy is ubiquitous in nature, therefore, radiation surrounds us and permeates us every day. Without realizing it, we are in contact with sources of radiation. Sources of radiation can be harnessed for beneficial effects such as used in diagnostic and healing treatment in nuclear medicine.
Another more general definition of radiation is: any combination of elementary particles (alpha, beta, etc.) that has sufficient kinetic energy to interact with and transfer energy to objects that intercept their path.
This concept of radiation is the basis for instrumentation and detection, and it is the foundation for the biological effects.
In contrast to radiation, we often refer to radioactivity. Radioactivity is a substance containing atoms that undergo radioactive decay. The nucleus seeks equilibrium by a statistical process of ejecting an energetic particle, like an alpha, beta, positron, x-ray, or gamma (x-rays originate from orbiting electrons and not the nucleus). This is an important distinction from radiation. Radioactivity is the material containing atoms that emit radiation as energetic particles.
