Nonlinear periodization is a method in which you constantly vary your training volume. This means you’ll have high, low, and moderate volume days in your weekly workouts. This method has been shown to be more effective at increasing gains in size and strength than linear periodization!
Reference: Simão, R., Spineti, J., de Salles, B. F., Matta, T., Fernandes, L., Fleck, S. J., … & Strom-Olsen, H. E. (2012). Comparison between nonlinear and linear periodized resistance training: hypertrophic and strength effects. The Journal of Strength & Conditioning Research, 26(5), 1389-1395.