Light amplification by stimulated emission of radiation (5)
I believe the answer is:
laser
'light amplification by stimulated emission of radiation' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for laser that I've seen before include "Light amplification by stimulated emission of radiation" , "type of surgery" , "Intense light generator" , "Coherent light beam" , "scientific instrument" .)