Crucially, the "mono-" bit means "one." One seller.
1 < n ⪅ 5 is "oligopoly".
If they collude and conspire it's a "cartel", but technically a cartel can also be a subset of players in a "polyopoly" (which is "many sellers", i.e. a healthy market). It's just way easier if the cartel = everyone :)
Crucially, the "mono-" bit means "one." One seller.
1 < n ⪅ 5 is "oligopoly".
If they collude and conspire it's a "cartel", but technically a cartel can also be a subset of players in a "polyopoly" (which is "many sellers", i.e. a healthy market). It's just way easier if the cartel = everyone :)