  We present a unified approach to describe spasing in plasmonic systems
modeled by quantum emitters interacting with resonant plasmon mode. We show
that spaser threshold implies detailed energy transfer balance between the gain
and plasmon mode and derive explicit spaser condition valid for arbitrary
plasmonic systems. By defining carefully the plasmon mode volume relative to
the gain region, we show that the spaser condition represents, in fact, the
standard laser threshold condition extended to plasmonic systems with
dispersive dielectric function. For extended gain region, the saturated mode
volume depends solely on the system parameters that determine the lower bound
of threshold population inversion.
