Linear: A differential equation is called linear if there are no multiplications among dependent variables and their derivatives. In other words, all coefficients are functions of independent variables.
Nonlinear: Differential equations that do not satisfy the definition of linear are nonlinear.
Quasilinear: For a nonlinear differential equation, if there are no multiplications among all dependent variables and their derivatives in the highest derivative term, the differential equation is considered to be quasilinear.
