A nonlinear differential equation is an equation that is not linear in the unknown function and its derivatives. In this case, the equation $y'' + y^2 = 0$ is nonlinear because the term $y^2$ is not linear in $y$.

The order of a differential equation is the highest order of the derivative that appears in the equation. In this case, the highest order of the derivative is 3, so the order of the equation is 3.

There are a variety of methods for solving nonlinear differential equations, including separation of variables, integrating factor, and variation of parameters. The choice of method depends on the specific equation being solved.

The general solution of the nonlinear differential equation $y' = y^2 + 1$ is $y = \frac{1}{2} \tanh^{-1}(x + C)$, where $C$ is an arbitrary constant.

The particular solution of the nonlinear differential equation $y'' + y = \sin(x)$ with the initial conditions $y(0) = 1$ and $y'(0) = 0$ is $y = \frac{1}{2} \sin(x) + \frac{1}{2} \cos(x) + 1$.

The logistic equation, Gompertz equation, and Verhulst equation are all types of nonlinear differential equations that are often used to model population growth.

The general solution of the nonlinear differential equation $y' = \frac{y}{x}$ is $y = Cx$, where $C$ is an arbitrary constant.

The Michaelis-Menten equation, Arrhenius equation, and Eyring equation are all types of nonlinear differential equations that are often used to model chemical reactions.

The particular solution of the nonlinear differential equation $y'' - y = \sin(x)$ with the initial conditions $y(0) = 0$ and $y'(0) = 1$ is $y = \frac{1}{2} \sin(x) + \frac{1}{2} \cos(x)$.

The SIR model, SIS model, and SEIR model are all types of nonlinear differential equations that are often used to model the spread of infectious diseases.

The general solution of the nonlinear differential equation $y' = y^2 - 1$ is $y = \frac{1}{2} \tanh^{-1}(x + C)$, where $C$ is an arbitrary constant.

The simple pendulum equation, damped pendulum equation, and driven pendulum equation are all types of nonlinear differential equations that are often used to model the motion of a pendulum.

The particular solution of the nonlinear differential equation $y'' + 2y' + y = \sin(x)$ with the initial conditions $y(0) = 1$ and $y'(0) = 0$ is $y = \frac{1}{2} \sin(x) + \frac{1}{2} \cos(x) + 1$.

The Navier-Stokes equations, Euler equations, and Bernoulli equation are all types of nonlinear differential equations that are often used to model the flow of fluids.

The general solution of the nonlinear differential equation $y' = y^3 + 1$ is $y = \frac{1}{2} \tanh^{-1}(x + C)$, where $C$ is an arbitrary constant.