Acoustic impedance in geophysical context What is the intuition behind acoustic impedance? Yes, I know that it is the opposition to a seismic wave travelling in the medium. But why it is defined as density &times; velocity?

The velocity of a seismic wave depends on density, so why not use only the velocity contrast or density contrast between layers instead of the impedance contrast?
This concept actually comes from the electrical. They define the impedance as $Z=\frac{V}{I}$. 
Where V is potential difference/ voltage and I is current.

In acoustics, a similar expression can be obtained using the plane wave equation. 
Since,
                $$\partial_t V = -\rho^{-1}\partial_xP$$
where $P$ is pressure and $V$ is velocity.
Now take fourier transform of both sides and defined impedance as $$Z(\omega)= \frac{P(\omega)}{V(\omega)} = \rho c$$
Where, obviously, $P$ is analougous to potential difference and $V$ is analougous to current.

You may refer to any wave theory book for details.