Unit of ultrasound A-scans
The amplitude is just arbitrary units for relative measurements only. The absolute values don't really matter all that much.

The peaks in the graph represent changes in tissue properties &mdash; specifically, changes in acoustical impedance, which cause energy to be reflected back toward the source. It is the horizontal position (representing distance) and width of these peaks that is more relevant than their amplitude. Indeed, you can see that some of them "saturate" the receiver &mdash; they have flat tops.