The phrase “X is not a dictionary” has been used in at least two contexts with distinct meanings.
WikiWikiWeb uses the phrase in the sense of “we shouldn’t try to have an entry for everything”. From “Wiki Is Nota Dictionary”:
Please, do not make pages for the words you use unless you are confident such a page would be valuable to this audience.
In addition, WardCunningham adds:
Wiki can be driven to pointless tangents by careless formation of hyperlinks. For example, say I use the phrase “Big Mac” in a contribution, alluding to the heavily marketed hamburger. Does Wiki need a definition of Big Mac? No way. Although some readers might benefit from a definition, in light of our AmericanCulturalAssumption(s), such a definition would be better included within the prose as above than as a footnote or hyperlink. Ask yourself: if the page you cite were to be created, would it serve Wiki well as a point of entry in this web of knowledge?
Wikipedia, on the other hand, uses the phrase in the sense of “a Wikipedia entry should say different things from what a dictionary entry says”. From “Wikipedia is not a dictionary”:
Wikipedia articles should begin with a good definition, but they should provide other types of information about that topic as well. The full articles that Wikipedia’s stubs grow into are very different from dictionary entries.
Each article in an encyclopedia is about a person, a people, a concept, a place, an event, a thing etc., whereas a dictionary entry is primarily about a word, an idiom, or a term and its meanings, usage and history.
For this site
For this site, “not a dictionary” means more closely to how WikiWikiWeb uses the term; in other words, this site is not meant to have an entry for every single idea in existence. Instead, I want to collect here my thoughts on some topics that interest me, and that I think would interest others as well.