The Collaborative International Dictionary of English v.0.48:

radioactivity \ra`di*o*ac*tiv"i*ty\
   (r[=a]`d[i^]*[-o]*[a^]k*t[i^]v"[i^]*t[y^]), n. (Physics)
   a form of instability which is a property of the atomic
   nuclei of certain isotopes, which causes a spontaneous change
   in the structure of the nucleus, accompanied by emission of
   energetic radiation. The radiation emitted is usually
   sufficient to cause ionization in matter through which it
   passes, and is therefore called ionizing radiation. The
   radiation emitted by most radioactive substances is one of
   three types: alpha rays, beta rays, or gamma rays. Some
   chemical elements have no stable isotopes, and these are
   referred to as radioactive elements, and the element itself
   is said to possess radioactivity.

   Note: The changes in radioactive nuclei which cause radiation
         in most cases cause the chemical identity of the
         nucleus itself to change, as when tritium (an isotope
         of hydrogen) emits a beta ray and converts to helium.
         The radioactive decay process is a first-order
         reaction, and the rate of decay of a particular isotope
         can therefore be expressed as the half life of the
         isotope, which is the time it takes for one half of the
         remaining undecayed isotope to decay, and is a constant
         independent of the proportion of original material
         which has already decayed. The half life of tritium,
         for example, is 12.3 years.
         [PJC]

WordNet (r) 3.0 (2006):

radioactivity
    n 1: the spontaneous emission of a stream of particles or
         electromagnetic rays in nuclear decay [syn: radiation,
         radioactivity]

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "radioactivity":
   activity, artificial radioactivity, contamination, curiage,
   decontamination, fallout, half-life, natural radioactivity,
   nuclear radiation, radiant energy, radiation,
   radioactive radiation, radiocarbon dating, radiolucency,
   radiopacity, radiosensibility, radiosensitivity, saturation point,
   specific activity