What you get when whitewood is on the menu varies with location.
Generally where I am in the west it is a softwood. It is called whitewood so they can throw whatever is cheap and available in the bin and not be concerned with naming the species correctly. Species can and will vary with each delivery. In the east where hardwoods outnumber the softwoods it can more often be some whiteish hardwood.
So the real answer to what is whitewood is, it depends.