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]

Source: The Collaborative International Dictionary of English v.0.48
 

radioactive adj : exhibiting or caused by radioactivity; "radioactive isotope"; "radioactive decay"; "radioactive fallout" [ant: nonradioactive]

Source: WordNet (r) 2.0
 

Radio-active \Ra`di*o-ac"tive\, a. [Radio- + active.]

(Physics) Capable of luminescence under the action of cathode rays, X rays, or any of the allied forms of radiation. -- Ra`di*o-ac*tiv"i*ty, n.

Source: Webster's Revised Unabridged Dictionary (1913)