Exposure is subjective and should be based on the intended tonality of your composition.
Overexposure is when the tonality of your picture seems to bright to you.
Underexposure is when the tonality of your picture seems to dark to you.
The right exposure is whatever you think the best exposure/tonality for your specific picture should be.
I think of overexposure and underexposure as negative terms when describing a picture. It means I missed the mark when choosing my exposure settings based on what I was metering off of.
I think of an increase or decrease in exposure as a thoughtful decision used in getting the exposure that I want.
I’ll usually intentionally increase or decrease my exposure. While an increase or decrease in my exposure makes my picture lighter or darker, that change in exposure is the right exposure for my picture.
Often times a picture looks better when you intentionally increase or decrease your exposure from the suggested exposure of your camera. That increase or decrease in exposure isn’t overexposure or underexposure, it’s the right exposure.