A stub is an article that is too short, but not so short as to be useless. In general, it must be long enough to at least define the article's title, which generally means 3 to 10 short sentences. Note that even a longer article on a complicated topic may be a stub; conversely, a short article on a topic of narrow scope may not be a stub.
Another way to define a stub is an article so incomplete that an editor who knows little or nothing about the topic could improve its content after a superficial Web search or a few minutes in a reference library. An article that can be improved by only a rather knowledgeable editor, or after significant research, may not be a stub.