What does pumping a transition mean in lasers?

[salsero]

What is the meaning of the term "pumping a transition" in relation to lasers?

 

[Integral]

The basic principle of a laser is that a significant number of electrons can be "pumped" to an unstable orbital. This is referred to as a populaltion inversion. To "pump a trasisiton" is to cause this population inversion to occur. When stimulated by a photon all of the inverted population cascades to a more stable orbital, thus a laser beam.