This page documents a Wikidata policy. It is a widely accepted standard that all editors should normally follow. Changes made to it should reflect consensus. When in doubt, discuss your idea on the project chat.
Note that the guidelines below are intentionally left a bit vague. In case of disagreement over the notability of an item, you can launch a request for deletion. The final decision is always up to the community.
Wikidata in its first phases has two main goals: to centralize interlanguage links across Wikimedia projects and to serve as a general knowledge base for the world at large. An item is acceptable if and only if it fulfils at least one of these two goals, that is if it meets at least one of the criteria below:
It contains at least one valid sitelink to a page on any Wikimedia Project.
To be valid, a link must not be a talk page, MediaWiki page, special page, user page, file, translations page, pages used by LiquidThreads (i.e. pages in Thread and Summary namespace), pages used by Flow (i.e. pages in Topic namespace) or portal subpage. Note that a single Wikimedia page cannot have more than one sitelink in Wikidata and that a sitelink cannot point to a redirect.
If a link is a subpage of a template, the item must contain at least two such sitelinks, and any of them mustn't be one of /doc, /sandbox, /testcases or /TemplateData subpages.
If a link is a subpage of a module, it mustn't be a /doc page.
In addition, an item with only a sitelink to a category page in Wikimedia Commons is not allowed on main article items. However, it's allowed to link Wikimedia Commons categories with categories in other Wikimedia sites in items.
On Wikisource, items for mainspace pages, Author pages, Translation pages, and Portal pages are valid, along with items for namespaces that exist on other Wikimedia sites (Category, Project...). Pages in the Index and Page namespaces are not considered valid. The status of subpages of mainspace pages (for example, individual chapters) is undetermined.
On Wikinews, Pages in the Comments namespace are not considered valid.
On Wikidata, items and properties are not allowed to link as sitelinks.
It refers to an instance of a clearly identifiable conceptual or material entity. The entity must be notable, in the sense that it can be described using serious and publicly available references. If there is no item about you yet, you are probably not notable.
It fulfills some structural need, for example: it is needed to make statements made in other items more useful.