Why is it called training volume if training volume only concerns duration and frequency? (2 parameters)
I’m confused because I thought volume required 3 parameters.
Maybe there is a nifty story as to why it’s called training volume and not training area.
Because volume also means an amount or quantity of something. Tis the English language.
If it only contains 2 parameters you are not doing enough of it to make a substantial amount (volume) 
Drafting geek alert. Training Volume is based upon time spent training, and frequency of training. In a sense, it creates a 2-axis coordinate system that can be uses to show “volume”.
I see time as the vertical axis (hours as units) and frequency as the horizontal (days as units). You can create a line graph between the time spent training on each day. When you do, you effectively create and “area” of training under that line. I could see calling this area “volume” and is a decent visual representation of the amount of training.
Crap, need more coffee… totally right there, doh! 
but seriously as @handynzl says… “the number or amount of something in general”
Solved my headache right here.
