Wikipedia:Wikipedia is not a dictionary
Part of What Wikipedia is not.
Wikipedia is not a dictionary, and an entry that consists of just a definition does not belong: But, an article can and should always begin with a good definition or a clear description of the topic. If you're interested in working on a wiki dictionary, check out the Wiktionary project!
It's natural, when you see Wikipedia for the first time, to mistake it for a dictionary. Many of the entries, at present, are brief. So some people start an article, listing several senses of a term, and then stop working on that article, as though they were interested only in giving the meanings of various terms. This can result in a stub article, or worse, a stub with no potential for growth and expansion.
This is not a habit to be encouraged.
Wikipedia is not a dictionary
Nearly everyone here agrees that in general, stub articles are to be encouraged. There are some differences of opinion as to whether just definitions are acceptable. If you want to make everybody happy, add a little encyclopedic information of some sort. Don't just give the meanings of the word. If you do just give the meaning of the word, nobody is going to be mad at you. They'll simply take the entry to be a "stub" article, which will be expanded later. That's probably OK, in most cases.
Moreover, there are plenty of senses of terms that aren't of interest in an encyclopedia. They would be, in a dictionary, but Wikipedia isn't a dictionary. So it makes no sense to describe those other, mere dictionary senses of terms in Wikipedia articles (unless, somehow it is important to describe those senses in order to clarify the main topic of the article).
Examples of articles that avoid being dictionary definitions by including "encyclopedic" information: Copra, Castanets, Fuck.
Now that that's out of the way, note that while Wikipedia is not a dictionary, our companion project Wiktionary is.
Wikipedia is not a usage guide
Wikipedia is not in the business of saying how idioms, etc., are used. (But, of course, it's often very, very important in the context of an encyclopedia article to say just how a word is used. E.g., the article on freedom will, if it doesn't already, have a long discussion about this.)
Articles that have been heavily cut to avoid becoming usage guides include gender-neutral pronoun and non-sexist language. Articles with information on how a word is used include singular they, homophobia, and sexism, SNAFU.
Wikipedia is not a slang or idiom guide
By a simple extension of the latter, Wikipedia is not a hacker/computer usage or other slang and idiom guide. We aren't teaching people how to talk like a hacker or a Cockney chimney-sweep; we're writing an encyclopedia. (See meta:Knocking her dead one on the nose each and every double trey for a historical example.) (But see jargon file; also, articles, even extremely in-depth articles, on hacker culture are very welcome, and insofar as guides to some particularly essential piece of hacker slang is necessary to understand those articles, of course articles on that slang would be great to have.)
Wikipedia is not a biographical dictionary
This is especially true in the case of biographical entries. There are special reference works known as genealogical or, more often, biographical, dictionaries. These tend to focus primarily on the immediate family connections (parents, spouses, children and their spouses) of the article subject. Wikipedia is an encyclopedia, and as such focuses more on the actions and contributions of an article subject. This means that many genealogical details may be omitted in exchange for a better-flowing, more rounded article.
Biography articles should only be given for people with some sort of achievement. A good measure of achievement is whether someone has been featured in several external sources. Minor characters may of course be mentioned within other articles (e.g. Ronald Gay in gay-bashing). While on the one hand we are all certainly delighted that Wikipedia is growing in breadth, some (but not all) of us view breadth at the expense of the very notion of what we are working on--an encyclopedia--as a bad idea.
See also wikipedia:what Wikipedia is not, Wikipedia:Find or fix a stub, Wikipedia:The perfect stub article, and Wikipedia:Things to be moved to Wiktionary