I think the full name is/was the PlayStation-X, but the X got dropped off somewhere along the line and only hangs around in the abbreviated PSX form. I'm not sure exactly where the "-X" stopped being used, but it might have been during the time when the system went from being the intended Super Nintendo CD-Rom add-on it was planned as to the stand-alone machine it eventually turned into.
On a tangential note, Marvel will be releasing new TPBs of the 1980s GI Joe comic. Choose life. Choose a job. Choose a career. Choose a family...Choose washing machines, cars, compact disc players and electrical tin openers.