Question

what is radioactivity

Answer 1

the emission of ionizing radiation or particles caused by the spontaneous disintegration of atomic nuclei.
radioactive substances, or the radiation emitted by these.

Answer 2

Radioactive decay is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is considered radioactive. Three of the most common types of decay are alpha decay, beta decay, and gamma decay, all of which involve emitting one or more particles or photons.