Stimulated Emission
 
Stimulated emission is the process by which an atomic electron (or an excited molecular state) interacting with an electromagnetic wave of a certain frequency may drop to a lower energy level, transferring its energy to that field.  
 
A photon created in this manner has the same phase, frequency, polarization, and direction of travel as the photons of the incident wave.  
This is in contrast to spontaneous emission which occurs without regard to the ambient electromagnetic field.  
 
See also:
Spontaneous Emission
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Source:
http://en.wikipedia.org/wiki/Stimulated_emission