A library to design and use Volterra models. Other information about Volterra models can be found here https://en.wikipedia.org/wiki/Volterra_series . If you want a more formal definition you can find more information about the Volterra theory and the operations in [1].
[1] M. Schetzen, “The Volterra and Wiener Theories of Nonlinear Systems”, Krieger Publishing Co., Inc., Mel- bourne, FL, USA, 2006.