Quasilinear PDE definition? Quasilinear PDE definition?

Here it's written that:


  Definition 3: A partial differential equation is said to be
  quasilinear if it is linear with respect to all the highest order
  derivatives of the unknown function.


What does mean to be linear with respect to all the highest order derivatives?

Does it mean that the terms containing the highest order derivatives are linear, but there might be other terms that are nonlinear?
Yes, that's it. The term
$$\frac{\partial u}{\partial y}\frac{\partial ^2u}{\partial y^2}$$
is linear with respect to $\frac{\partial ^2u}{\partial y^2}$, but the term
$$\left(\frac{\partial ^2u}{\partial y^2}\right)^2$$
is not linear.

If you have a second order derivative, it does not matter if the equation has a term like $u^2$ to be quasilinear. You just need to check the highest order derivatives (second order ones in the examples of the link).
The simplest definition of a quasi-linear PDE says:


  A PDE in which at least one coefficient of the partial derivatives is really a function of the dependent variable(say u).


For example,

$\mathrm{\frac{\partial^2u}{\partial x_1^2} +u\frac{\partial^2u}{\partial x_2^2} =0}$