There are two distinct definitions of natural.
Under the narrower definition, a natural object is one that occurs often in random patterns. There is no precise measure of naturalness, since the most useful definition of "random" in this context is open to debate; nonetheless, objects such as blocks, blinkers, beehives and gliders are very natural, while eater 2s, darts, guns, etc., are not.
Less strictly, an object is said to occur naturally if it has been found in the ash of an asymmetric random soup; the object in question may then also be said to be natural, e.g. "a natural Coe ship". Objects found in the ash of symmetric soups are sometimes said to occur semi-naturally or almost naturally.