A 'Natural Move' is most often used to refer to a move which accords with basic principles, and is therefore intuitively the kind of move which is appropriate for the given position.
Although a move may look natural, that does not necessarily make it a good move, since every position must be judged on its own merits.
The opposite, an 'unnatural move' is more commonly called a counter-intuitive move. This is a move which seems to break basic principles and which common sense would suggest is bad.
Similarly, a counter-intuitive move does not necessarily have to be a bad one.
Computers often find strong counter-intuitive moves that humans struggle to see. Conversely, humans can play 'natural moves' by using their experience, which computers cannot.
An example of a natural move which turns out to be bad:
Example source: Chessbase