Question

What does amplitude measure?

Answer 1

Amplitude measures the intensity of a wave. On a wave diagram, it is the distance "`a`" from the vertical middle to the crest or trough of the wave:

http://www.bbc.co.uk/

In terms of sound, it is how loud the sound is:

https://physics.tutorvista.com/waves/

In terms of light, it is how bright the light is:

http://slideplayer.com/slide/8293714/

In terms of electron orbital probability densities, it is the relative probability electrons are found in those radial coordinates (black is, it can be found more often):

http://cronodon.com/Atomic/