I heared the terminology in Diablo2 2003/2004 while the ladder was known and the game was the biggest thing ever for a few years.
You either played on the ladder or you played casually and that just meant less than X hours a day.
It still remains true for WoW.
It doesnt matter if you play arena, m+, mythic-raiding or what ever. If you participate at any level of the game for a very short amount of time, you are a casual in any MMO.
Nobody really cares for short term childrens play in this game. Do what ever you enjoy, but be aware you play an MMO, the endless grind game genre, where grinding or collecting is in the end all what matters.