I think in the olden days the surrounds speakers in a Dolby Surround system (circa 1982) were often referred to in the press, and manuals, as rears. And it just stuck. Now with the move to immersive systems I too would rather use E.S.O to describe the positions. E => number of ear level speakers. S => number of Subs. O => number and positions for the overhead speakers. I'm a KISS sort of guy (Keep It Simple Stupid).