Is it littering to throw out an apple core?

Yes it is littering. It is your garbage that you are tossing some where other than an approved location or in an approved manner. Your old apple core did not come from an apple grown from a tree in the middle of your sidewalk, edge of the street, median, or park. So, you would not be throwing it out into nature where it came from. Whether you threw it in these places, or in your back yard, chances are there is a local ordinance, code, or law that prohibits you from simply tossing your apple core, bread crust, or other food. This is due to disease, insect, four legged, and winged pest reasons.

On the other hand, it is bio-degradable and potential food for a number of critters including worms and soil fungi. So, environmentally it is not the same as tossing your candy wrapper which is more likely than not to be pastic based. Nor is it as environmentally damaging as any one of a number of other items; most of which are comprised of non-organic materials or contain a coating of a non-organic material. On the other hand, too much of even a good thing can be a pollutant and/or contaminate. - What would happen if 100 people all tossed their apple cores into the same bushes you threw yours? What if every family in your town threw just one apple core into those same bushes? You get the idea.

Exact definitions and laws vary by location.

Commonly throwing anything on the ground not already naturally in that specific location is littering. Unless you pick up a leaf or rock already there and put it back down, it's littering. If it's something you'd throw in the trash at home, it's littering.

Would you like your front yard to be everyone's apple core and bread dumping ground, because it's "natural." No? It's littering.

Items thrown out like that attract insects, take time to decay, attract bacteria, feed animals who don't belong there, possibly grow non-native trees, etc. Of course it's littering.

It may vary by city. However, my city defines litter as "including" certain items which themselves are non-biodegradable, but is not exhaustive definition. The act of littering is defined in such a way that it may be subjective, because it includes the idea that the item being left behind would contribute to the "diminution of property values, prevent crime and preserve the public health, safety and welfare" of the area. If a bioderegradable item is left behind on the ground, it could pose a pedestrian hazard, for example if someone were to slip and fall. Or it could pose a health hazard if it attracted dengue or zika carrying mosquitoes or yellow jackets. So even though the definition doesn't explicitly state that your apple core is litter, because liitering is partially defined as something that causes this vague hazard category, you could be cited and wouldn't have any legal grounds for defense. I imagine that a lot of littering statutes are similarly defined.

Yes. It's littering.

Of course it is littering, encouraging rats to the area and generally making the place look like a tip, find a bin or take it home!