Merriam-Webster Collegiate Dictionary - radiation

 noun  Date: 15th century  1.  a. the action or process of radiating  b. the process of emitting radiant energy in the form of waves or particles  c.  (1) the combined processes of emission, transmission, and absorption of radiant energy  (2) the transfer of heat by ~ — compare conduction, convection  2.  a. something that is radiated  b. energy radiated in the form of waves or particles  3. radial arrangement  4. adaptive ~  5. radiator  • ~al adjective  • ~less adjective  • radiative adjective