Wikipedia:Content that could reasonably be challenged

Good article criteria and Did you know/Guidelines refer to "content that could reasonably be challenged". This concept is in some ways similar to Verifiability's concept of "likely to be challenged", but is a fair bit broader. That is to say, some things that are not "likely to be challenged" may still "reasonably be challenged". "Likely to be challenged" means a greater-than-50-percent chance of an editor requesting a citation in good faith, and is a rough proxy for whether a given claim is at all controversial. "Could reasonably be challenged" means that, were an editor to request a citation, this would be a reasonable exercise of editorial discretion, rather than pedantry or bureaucracy. This is more a definition of exclusion: It applies to all but the most obvious claims.

The "could reasonably be challenged" rule only ever supplements other citation rules. The minimum referencing requirements must always be met, as must specialty requirements such as those regarding living people and those regarding biomedical topics.

What might be unreasonable to challenge?
There is no community-wide consensus as to when a claim is unreasonable to challenge, and there are some editors who feel that all claims can be challenged. However, the request for comment that resulted in the "could reasonably be challenged" rule identified two main categories that many participants did not see as always requiring citations:

However, whether a claim is reasonable to challenge will always be a case-by-case determination. Sometimes, what is perceived as reasonable to challenge may depend on context. Editors of an article about the Australian military may feel it obvious that the governor-general is the commander-in-chief, whereas if that fact comes up in another topic area, a citation might be expected for it. Generally, once a claim has been challenged, a citation is provided regardless. Furthermore, editors may always proactively provide citations even for claims that are unlikely to be challenged; this is the de facto practice of many writers of good and featured articles.
 * Widely-known and non-controversial facts, for instance:
 * Snow is cold to the touch
 * Giraffes are tall
 * Facts that can be trivially verified in widely accessible sources, especially when mentioned in passing, for instance:
 * Basic geographic information, like that China is north of Vietnam
 * The definitions of words, including:
 * Paraphrasing a cited source. One can paraphrase as "Jones was a friendly person, according to Smith" without needing to cite the definition of "amiable".
 * Describing the source itself. One can call a block of lines in a poem a stanza, even if no source uses that term specifically about that poem.
 * Brief, non-controversial descriptions of entities, especially well-known ones—for instance, that the Federal Bureau of Investigation is a U.S. government agency, or that Marie Curie was a physicist and chemist.

What might be reasonable to challenge?
Short answer: Most claims.

Long answer: All claims that are "likely to be challenged", as defined by that supplement, are by definition reasonable to challenge. Furthermore, there are many statements that may not be prone to challenge, but which it would still be reasonable to request a citation for. For instance:
 * The Moon is the Earth's only natural satellite: While it is common knowledge that the Moon orbits the Earth, a reader might reasonably wonder whether any other natural body does.
 * Wikis can be edited collaboratively: This is generally true, but exceptions exist, so it is reasonable to request a citation.
 * France borders Brazil: This can be seen on any map if one knows where to look, but requires the reader to understand the political status of French Guiana, which is non-obvious, and thus may lead to a reasonable request for a citation

Under Good Article Criterion #2b, all of these claims need to be cited inline, even if they do not meet the bar of "likely to be challenged", and whether or not someone has explicitly challenged them. In other words, any unsourced claims in a good article must be ones that it would be unreasonable to insist on a citation for.