2 definitions found From WordNet (r) 2.0 [wn]: radioactive adjective: exhibiting or caused by radioactivity; "radioactive isotope"; "radioactive decay"; "radioactive fallout" [ant: {nonradioactive}] From The Collaborative International Dictionary of English v.0.44 [gcide]: radioactive \ra'di*o*ac"tive\ (r[=a]'d[i^]*[-o]*[a^]k"t[i^]v), a. [Radio- + active.] 1. (Physics) Capable of luminescence under the action of cathode rays, X rays, or any of the allied forms of radiation. [obsolete] [Webster 1913 Suppl.] 2. (Physics) of, exhibiting, or caused by {radioactivity}. [PJC]

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