register
How high or low the notes sit overall.
How high or low the music sits overall, from sub-bass to piercing highs. The same melody moved up or down an octave changes register without changing its note names, and register is where mud (too much low) and thinness (too much high) come from. Mixing is largely giving each part its own register.
Learn it: Pitch and the octave
The one-sentence definition at the top is the same one the Theory Aide extension shows as a tooltip inside Ableton Live. Hover any dotted term for its own.