TIL Option[_] isn't a higher-kinded type. It's just a type constructor. Higher-kinded types are types that abstract over type constructors (so X[F[_]]). As known as 1st order & higher-order abstraction. But best refer to the latter as "type constructor polymorphism".
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Or in other words a higher-kinded type has this shape: (* -> *) -> * Anything I'm missing?
Only that higher-kinded types include more complex examples, too, e.g. the kind of a type which could be parameterized on a functor.