A world is a large, more-or-less self-contained expanse, usually but not necessarily populated. The criteria for being "self-contained" are somewhat less stringent than that of a plane; for instance, planets in the same plane are generally considered separate worlds, since travel within a planet is much easier than travel between planets, and most inhabitants of a typical planet have no easy way to visit other planets. In fact, planets and analogous bodies—"mounds", in the æalogical terminology—are what the word "world" is most commonly used to refer to. It has, however, also been used in reference to planes—though usually not to universes, but rather to planes that are more or less uniform in content and nature—and to various other areas within planes or collections of planes, up to and (rarely) including entire cosmoi. On the other hand, there being no specific cutoff regarding how large an area must be to qualify as a "world", some writers have used the word to refer to areas as small as countries, or even cities, especially if they regard those areas as being particularly isolated or self-sufficient.
Because of its subjectivity and vague definition, the word "world" is not generally used as a technical æalogical term; æalogists prefer to refer to the topics of their discussion more specifically, as planes, mounds, et cetera. Nevertheless, there are two phrases containing the word that are widely used in æalogical circles. First, mounds are often referred to as "planetary worlds". Second is the phrase "alternate world", which can refer to any alternate plane or part of a plane. Some æalogists dislike even these usages, however.
In general, the word "world" can often, but not always, be taken to refer to a planetary world. Usually, however, the specific meaning will be clear from the context.