The fraction of a number of atoms of a radioactive nuclide that disintegrates in a unit of time. The decay constant is inversely proportional to the radioactive half-life
Centers for Disease Control and Prevention, Emergency Preparedness and Response, Radiation Emergencies, Radiation Dictionary, U.S. Department of Health and Human Services, Atlanta, GA, USA 2011. http://emergency.cdc.gov/radiation/glossary.asp, (Dzstr/ Rad Emr - Amer) {820}