Radiation (in physics), an emission of energy or particles.

Radiation (medicine), characteristics of e.g. pain to spread to other parts of the body than the origin.

Radiation in evolutionary biology:

Evolutionary radiation (to which Radiation (biology) redirects), a diversification into several lineages from a common ancestor, and

Adaptive radiation, an evolutionary radiation to fill many ecological niches. Often especially rapid, produces a wide range of apomorphies, and is commonly found when new habitat such as on oceanic archipelagos is colonized.