Incompressible and compressible flow

Have you ever been confused which one is compressible or incompressible flow in fluid dynamic course ? The importance of these flows is mentioned in the Bernoulli's equation, which stated that it is valid only with incompressible flow.

I used to think that compressible flow is the flow of gas, and incompressible flow is the flow of liquid. As everyone knows gas is compressible but liquid is incompressible, that's why I came up with that idea.

But I was wrong. It turns out that flow of gas could be incompressible flow too, and it is usually so!, because most flows are well below the speed of sound. That means Bernoulli's equation can be applied for both liquid and gas flows. Check this out from Wiki:

For flow of gases, to determine whether to use compressible or incompressible fluid dynamics, the Mach number of the flow is to be evaluated. As a rough guide, compressible effects can be ignored at Mach numbers below approximately 0.3. For liquids, whether the incompressible assumption is valid depends on the fluid properties (specifically the critical pressure and temperature of the fluid) and the flow conditions (how close to the critical pressure the actual flow pressure becomes).

So this is just a reminder, at least for myself.