Radioactivity is the process of generating radiation.
So when you say “that object is radioactive”, that means it’s generating radiation.
“Radiation” is a collective term for a number of different types of energetic particles that are given off by radioactive materials.
EDIT: This answer is only in the “radioactivity” context. See Chromotron’s comment below, “radiation” can refer to many other things too, which *don’t* come from “radioactive” sources.
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