File captions are a feature part of Structured data (the first part to be enabled). They are meant to store short, multilingual descriptions about files. See Help:File captions for more information. They were enabled on Wikimedia Commons in January 2019.

There are technical requirements (limited to 250 characters in length, and cannot contain markup) which necessarily make captions short.

• Captions are meant to be simple and short. While descriptions can be very expansive, captions are limited to 250 characters in length, cannot contain markup (like Wikitext links or HTML) and do not support external links.
• Captions are available as part of our structured data, while descriptions are not. In technical terms, a description is plain wikitext wrapped in a language template, wrapped again in an Information template ; while captions are Wikibase labels. As such, they will be searchable through the API, making it easy to find/filter/pull captions from files as metadata.

In many cases, the caption will be similar or identical to the description (or even the file title!). For example, File:Fire station Hallstatt - October 2017 - 01.jpg has the English caption "Hallstatt fire station in October 2017".

Conversely, on Pluto-01 Stern 03 Pluto Color TXT.jpg there is a very extensive description (858 characters, and links) and the caption is much shorter (more or less the same caption used on the Wikipedia articles using that image).

Captions are stored on Wikimedia Commons, as part of Wikibase (and thus not as part of the wikitext). They are not stored on Wikidata. While Wikibase is indeed powering Wikidata, it is also deployed here on Wikimedia Commons.

If you are a logged in user the languages displayed are the ones listed in Babel boxes on your userpage.

You may enable the following gadgets in your preferences (under “Interface: Files and categories”):

• To collapse the caption block and add a link to expand it when needed: Collapse Captions (main script code, additional style code)
• To make the captions block more compact: Compact Captions (code)
• To hide the captions completely: Hide Captions (code)

Collapse Captions and Compact Captions can be combined if desired.

See in addition section Styling below.

The captions form field can be styled with Cascading Style Sheets. Add the rules into your common.css file or a browser addon like Stylus Userstyles Manager or xStyle. In the latter case you should use the provided restriction to Commons domain commons.wikimedia.org.

With the gadget Collapse Captions you will in collapsed mode by default only see an empty box apart from the Expand string. The following will make the caption visible in this state (i.e. Captions in your language):

Notes:

• Because of the method, how the line numbers are inserted here in the presentation, in the copy of the rules you will see every second line being empty. You can safely delete these empty lines, if you want. There is actually some indentation with a tab character, too, which is also not shown here.
  This is valid for the following rules with line numbers below, as well.
• Line 1: In tests the part !important was necessary. This will provoke a warning here in Commons in the code editor of common.css. Maybe it could be even left out when this file is used (tested only with browser addon).
• Line 2: This will remove the header (Structured data in your language). If you want to have it displayed instead, delete this line including the comma.

• Lines 10–12 are for formatting of the visible caption. If you want to have the default upright, bold string, delete these lines.
• Lines 13–14: You probably need in most cases only either of these two rules, though it doesn’t matter if you copy them both. To decide which one you usually will need look at their beginning:
  • .ltr: left-to-right writing systems like Latin script (e.g. English).
  • .rtl: right-to-left writing systems like Arabian script.

If you want to have the default string at beginning (left for ltr, right for rtl systems), delete these lines.

• Line 15 is only needed for rtl systems. It sends the toggle for collapsing and decollapsing to the left of the page. This rule will be active also for every toggle on all other Commons pages.

Further styling just for appearance, add this below the rules from above:

Notes:

• Line 1: If the ellipsis inside of content: " …"; is not in the usual style of your language then replace it according to your taste, but do not omit the quote signs! If the added content is on the wrong side in your writing system then replace after with before.
• Lines 2–5 are for the caption in uncollapsed mode. If you don’t want it to be underlined then delete line 4.

… but not that compact like with the gadget Compact Captions

Notes:

• If the block interferes with the preceding text block, then delete line 6 or at least reduce the amount of the value. margin-top: -0.5em; seems to be usable.
• Lines 1–3 can be left out if you use the unchanged rules from above, but it does not cause harm if you add them twice, as long as you remember this in future edits.

Further styling just for appearance:

Notes:

• Lines 2–3: Read notes above for .ltr and .rtl.
• Lines 4–5: If you do not want to have parentheses around the edit string (Edit in your language) either delete both rules or replace them with your preferred characters inside of content: "…";, but do not omit the quote signs!