A seed is an article which is clearly 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 a longer article may be a seed if the topic is complex enough; conversely, a short article on a topic which has a very narrow scope may not be a seed.
Another way to define a seed is an article so incomplete that an editor who knows little or nothing about the topic could improve its content after a superficial internet 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 seed.
Sizable articles which lack wikification or copy editing are generally not considered seeds. Note that small articles with little information may end up being nominated for deletion or be merged into another relevant article.
Once a seed has been properly expanded and becomes an article rather than just a seed, you should remove the seed tag from it. When writers begin a new article, they use the word seed to mean that it is still very short and that people can add a lot more useful information. Most seeds are only one paragraph or sometimes they are only a few lines. Seeds do not contain a complete description of a subject.
If you make or find a seed, add this special text to ask other users to make the article longer. The easy way to do this is to use the short-cut: [[Include(Seed)]]
Adapted from the Wikipedia guide on stubs: Wikipedia:Stub
For a list of seed pages you can improve on the Pawtucket wiki, see Seed Pages List.