A featured article exemplifies Wikipedia's very best work and is distinguished by professional standards of writing, presentation, and sourcing. In addition to meeting the policies regarding content for all Wikipedia articles, it has the following attributes.
It is:
well-written: its prose is engaging and of a professional standard;
comprehensive: it neglects no major facts or details and places the subject in context;
well-researched: it is a thorough and representative survey of the relevant literature; claims are verifiable against high-quality reliable sources and are supported by inline citations where appropriate;
stable: it is not subject to ongoing edit wars and its content does not change significantly from day to day, except in response to the featured article process; and
a lead: a concise lead section that summarizes the topic and prepares the reader for the detail in the subsequent sections;
appropriate structure: a substantial but not overwhelming system of hierarchical section headings; and
consistent citations: where required by criterion 1c, consistently formatted inline citations using footnotes—see citing sources for suggestions on formatting references. Citation templates are not required.
Length. It stays focused on the main topic without going into unnecessary detail and uses summary style where appropriate.
Professional, outstanding, and thorough; a definitive source for encyclopedic information.
No further content additions should be necessary unless new information becomes available; further improvements to the prose quality are often possible.
Prose. It features professional standards of writing.
Lead. It has an engaging lead that introduces the subject and defines the scope and inclusion criteria.
Comprehensiveness.
(a) It comprehensively covers the defined scope, providing at least all of the major items and, where practical, a complete set of items; where appropriate, it has annotations that provide useful and appropriate information about the items.
(c) In length and/or topic, it meets all of the requirements for stand-alone lists and includes at minimum eight items; does not violate the content-forking guideline, does not largely duplicate material from another article, and could not reasonably be included as part of a related article.
Structure. It is easy to navigate and includes, where helpful, section headings and table sort facilities.
Style. It complies with the Manual of Style and its supplementary pages.
(a) Visual appeal. It makes suitable use of text layout, formatting, tables, and colour; and a minimal proportion of items are redlinked.
Stability. It is not the subject of ongoing edit wars and its content does not change significantly from day to day, except in response to the featured list process.
The article is well organized and essentially complete, having been reviewed by impartial reviewers from a WikiProject or elsewhere. Good article status is not a requirement for A-Class.
Provides a well-written, clear and complete description of the topic, as described in Wikipedia:Article development. It should be of a length suitable for the subject, appropriately structured, and be well referenced by a broad array of reliable sources. It should be well illustrated, with no copyright problems. Only minor style issues and other details need to be addressed before submission as a featured article candidate. See the A-Class assessment departments of some of the larger WikiProjects (e.g. WikiProject Military history).
Very useful to readers. A fairly complete treatment of the subject. A non-expert in the subject matter would typically find nothing wanting.
Expert knowledge may be needed to tweak the article, and style issues may need addressing. Peer-review may help.
it contains a list of all references (sources of information), presented in accordance with the layout style guideline;
reliable sources are cited inline. All content that could reasonably be challenged, except for plot summaries and that which summarizes cited content elsewhere in the article, must be cited no later than the end of the paragraph (or line if the content is not in prose);
Useful to nearly all readers, with no obvious problems; approaching (although not equalling) the quality of a professional encyclopedia.
Some editing by subject and style experts is helpful; comparison with an existing featured article on a similar topic may highlight areas where content is weak or missing.
The article is mostly complete and without major issues, but requires some further work to reach Good Article standards. B-Class articles should meet the six B-Class criteria.
The article reasonably covers the topic, and does not contain obvious omissions or inaccuracies. It contains a large proportion of the material necessary for an A-Class article, although some sections may need expansion, and some less important topics may be missing.
The article has a defined structure. Content should be organized into groups of related material, including a lead section and all the sections that can reasonably be included in an article of its kind.
The article is reasonably well-written. The prose contains no major grammatical errors and flows sensibly, but does not need to be of the standard of featured articles. The Manual of Style does not need to be followed rigorously.
The article contains supporting materials where appropriate. Illustrations are encouraged, though not required. Diagrams, an infobox etc. should be included where they are relevant and useful to the content.
Readers are not left wanting, although the content may not be complete enough to satisfy a serious student or researcher.
A few aspects of content and style need to be addressed, and expert knowledge is increasingly needed. The inclusion of supporting materials should also be considered if practical, and the article checked for general compliance with the manual of style and related style guidelines.
The article is substantial, but is still missing important content or contains a lot of irrelevant material. The article should have some references to reliable sources, but may still have significant issues or require substantial cleanup.
More detailed criteria
The article is better developed in style, structure and quality than Start-Class, but fails one or more of the criteria for B-Class. It may have some gaps or missing elements; need editing for clarity, balance or flow; or contain policy violations such as bias or original research. Articles on fictional topics are likely to be marked as C-Class if they are written from an in-universe perspective.
Useful to a casual reader, but would not provide a complete picture for even a moderately detailed study.
Considerable editing is needed to close gaps in content and address cleanup issues.
An article that is developing, but which is quite incomplete and, most notably, lacks adequate reliable sources.
More detailed criteria
The article has a usable amount of good content, but it is weak in many areas, usually in referencing. Quality of the prose may be distinctly unencyclopedic, and MoS compliance non-existent; but the article should satisfy fundamental content policies such as notability and BLP, and provide enough sources to establish verifiability. No Start-Class article should be in any danger of being speedily deleted.
Provides some meaningful content, but the majority of readers will need more.
Provision of references to reliable sources should be prioritised; the article will also need substantial improvements in content and organisation.
The article is either a very short article or a rough collection of information that will need much work to become a meaningful article. It is usually very short, but if the material is irrelevant or incomprehensible, an article of any length falls into this category.
Provides very little meaningful content; may be little more than a dictionary definition
Any editing or additional material can be helpful. The provision of meaningful content should be a priority.