Wikipedia:Establish context

One of Wikipedia's rules to consider:

Almost every entry in the Wikipedia should begin with a line or two that establishes its context. These opening lines should include the most basic facts about the subject, including a short definition. Because Wikipedia is not a long, ordered sequence of carefully categorized articles like a paper encyclopedia, but a collection of randomly-accessible highly interlinked ones, each article should contain links to more general subjects that serve to "categorize" the article.

This very article, for example, begins by establishing that this article descibes one of Wikipedia's guidelines, and links to a list of them.

For example:

In mathematics, a quadratic equation is ...

In biology, ....

An article on Charles Darwin, likewise, should not begin with something like "Darwin created controversy with the publication of Origin of Species..." It should begin with:

Charles Darwin (1809 - 1882) was a 19th century biologist who proposed the modern theory of evolution....

(Wikipedia:Manual of Style has more on the specific format for biography articles; the authoritative format is there, this is just an example)

Establish fiction

Another example is fictional characters and places. Readers might not know, for instance, that Homer Simpson is not a real person. Start with, for example, "Homer Simpson is a fictional character in the television series..."

more on this: Wikipedia:Check your fiction.

No pages are subpages

A common context problem is writers linking a work from an author's page, and then starting an article with "A is his third novel..." without stating whose novel it is.

See debate.