This document discusses linear and nonlinear systems. It defines a linear system as one that satisfies the principles of superposition and homogeneity or scaling. The principles of homogeneity and superposition for a linear system are defined. Homogeneity means that scaling the input scales the output by the same factor. Superposition means that the output for the sum of two inputs is the sum of the outputs for each input individually. A linear system thus satisfies the property of linearity, where the output of scaled and summed inputs is the scaled and summed outputs. Nonlinear systems do not satisfy these properties.