Question

What do you understand by the term loudness w.r.t. sound waves?

Answer 1

The loudness of a sound wave is the intensity of the wave. The intensity of a sound wave is proportional to the square of the amplitude of the wave ($I\propto A^2$). The more the amplitude, the more will be the intensity, and hence more the sound will be loud.

the intensity of a sound wave is measure in terms of $decibel$.