The
lasing threshold is the lowest excitation level at which a
laser's output is dominated by
stimulated emission rather than by
spontaneous emission. Below the threshold, the laser's output power rises slowly with increasing
excitation. Above threshold, the slope of power vs. excitation is
orders of magnitude greater. The
linewidth of the laser's emission also becomes orders of magnitude smaller above the threshold than it is below. Above the threshold, the laser is said to be
lasing. The term "lasing" is a
back formation from "laser," which is an
acronym, not an
agent noun.