Radiation is most loosely defined as the transmission of energy through some form of medium or space. So for example, radio waves, a form of energy, can pass through completely empty space from point A to B. Note that electromagnetic radiation (e.g. radio, visible light, UV) does not require a medium. Another form of radiation is that of particle radiation, as would be the case when unstable atoms decay and release particles into space or the medium that surrounds them. For example, alpha radiation is the decay of a source particle such that it releases an alpha particle (2 neutrons + 2 protons).
Something is radioactive when a material source actively releases radiation. Put differently, it is actively giving off radiation. For example, an unstable atom with lots of energy might undergo ...
Definitions and examples for radiation, radioactive and ionizing radiation, as well as a comparison between stochastic and deterministic effects of ionizing radiation and their relation to dosage of radiation.