I admit that I have trouble with this new nomenclature by initials.
Grand Slam is Grand Slam.
Admit "CGS" for "career grand slam " but Y for year is unnecessary since Grand Slam involves in the year.
You're correct that a Grand Slam is originally what is nowadays referred to as a Calendar Year Grand Slam. Not sure when Grand Slam was no longer used when referring to winning all 4 tournaments during the same year, but these days the problem is that Grand Slam is also used when talking about simply any of the 4 tournaments. I wonder if they were initially referred to as Majors, and a Grand Slam was then winning all Majors during a single season? edit. I suppose they were called either majors or Grand Slam Tournaments, but these days people have omitted the 'tournament' part, and simply talk about someone winning a Grand Slam even if it's just one of the 4 tournaments.CYGS is to differentiate it from career grand slam (CGS). And this is not too new, is at least since Agassi won it in 1999 they created the other term AFAIK and changed the abbreviation for the calendar (year) grand slam.
I like Calendar Slam over Calendar Year Grand Slam. More concise.