Unary is another way of saying "of arity one". A one parameter function is said to be unary, whereas a two parameters function is binary, three is ternary and so forth...
The same principle applies to types. List[A] or Set[A], for example, takes one type A to form a new proper type, whereas Map[A, B] takes two and is said to be a binary type constructor.
On a side note, types which abstract over a type constructor are called higher kinded types (e.g : Monad[F[_]])
