Volterra series is similar to "power series" in calculas.
~1980, people use this pertubation mehod to predict
behaviors of slightly non-linear circuits like:
VCO, PA, mixer,...
Volterra series is more powerful than power series as it can also include
the memory effect by including them in the phase information while power
series only includes the nonlinearity coming from the amplitude variation.
Volterra series is a frequency domain method to model nonlinear components and it is imlemented in some nonlinear simulators. It does the same things as Harmonic balance, but it is baing base on a series it has no convergence problems.
It is very fast and powerful for circuits with slightly nonlinear behavoiur.