It's a good question and definitions vary. My working, broadest definition is "That which exists / that which occurs if humans do not use their intellect and ability to cause something else to exist / happen."

Starvation is natural. Death by predator is natural. Death due to exposure is natural. Death due to disease is natural. Tornadoes are natural. A biosphere-disrupting asteroid impact is natural. There are other things that are natural that are positives too, but it's good, I think, to keep in mind that "natural" doesn't always imply "good."