In short; a club... but, mostly a restricted club. There are levels of restriction where only a member can indulge along with certain lesser restricted rules where a friend of a.member was a guest and it could even get a bit more complicated but, mostly known as being where not anyone can simply walk in.

Carlos Pascoal covers it all in an excellent archived article here pertaining to the structure of some establishments.

My reason for mentioning it in my post as an unknown find was only to point out that, in most cases a casino name seemed to be widely listed in publications where not so much a club and, one of the reasons collectors should be aware since we are mostly interested in the chips used in gaming and matching them to a location or, in some cases, an operator who may have had only a monogram.