com defines natural as conforming to the usual or ordinary course of nature. To properly define such a complex word you must define it not only with what the literal meaning is, but by citing examples and interpreting the word yourself. The word natural to me means that something is not acquired, yet inherent. .
Natural is when a new born baby knows how to breath for the very first time. He or she somehow knows exactly what to do to pull in that first breathe of air. When an animal knows instinctively how to defend itself from being harmed; that is natural. Advertisements always talk about how their drug products are all natural. What they mean is that it has not been tampered with or altered in any way. Other words that have similar definitions that might help you properly define the word natural would be usual, normal, and ordinary. But knowing the definitions of these other easier to define words don't help to properly define natural.
Comparing this word with common analogies might help to properly define natural. The movie Basic Instinct's title was supposed to imply that murder is human nature. As in it is human instinct and not altered through culture conditioning. So instinct is also a synonym. Any natural history museum is called what it is because all their displays are of items that are natural. These items mean anything that isn't tampered with by its surroundings. All of these things better help describe what the meaning is to natural.
Some people will only by all natural foods and drugs. There are stores dedicated to better serving this public. Whole Foods only sells natural items. This would be anything without preservatives. Preservatives alter the natural way of life by keeping things the way they are supposed to be. Again this helps better understand the definition of natural because things that are the way they were supposed to be are considered natural. The phrase "naturally speaking" is used when someone is conversing in an ordinary tone and dialogue.