Does anyone know why instrumental tracks are also called "minus one tracks"?
"Music Minus One" is a trademark of a company that makes accompaniment tracks. They have been in business since 1950.
Accompaniment tracks that are not made by this company are colloquially referred to as "minus one tracks".
Let me explain this at a more basic level.
Let's say you have a piece of music that has four parts: a vocal part, a piano part, a bass part, and a drum part.
If you then remove one of the parts, you have a three-part arrangement.
Four parts minus one part equals three parts.
Accompaniment tracks are called "minus one" tracks because one part has been subtracted. There is a part that is missing. You use this type of recording because you want to record your own missing part onto it.