Optional Means Union With None

typing.Optional[X] is not a synonym for "this parameter is optional" (has a default value); a parameter can have a default without being Optional, and an Optional parameter can still be required. Optional[X] describes one thing only: whether None is an acceptable value.

The documentation defines it as exactly one type expression: typing.Optional[X] is equivalent to typing.Union[X, ____]. What single name goes in the blank?