When you are watching TV and you turn the volume up or down with the remote control say from 17 to 16, 9 to 10, etc., how do you describe that single amount of volume? Is the word notch correct to describe this single amount of volume i.e. turn the volume down a notch, turn the volume up a few notches?
Calling it a notch would be fine.
The scientific unit of measure is a decibel, but that isn't something you would say unless you're talking to someone who knows what it is, and you know how that correlates to the gradations on the remote control.