An interwiki link is a special link from a page on the English Wiktionary to a similar page on another Wikimedia project. These links do not show up in the page itself, but are displayed in a toolbar to the left (on mobile devices, it may be elsewhere).
Interlanguage links consist of the language prefix and then the name of the page (i. e. ]. If you want to link to a different Wiktionary inline in the form of a blue link instead of an interwiki, for example on discussion pages, add a colon (:) directly after the two left square brackets (] (which becomes de:word).
You can also link to other projects of the Wikimedia Foundation using their respective prefixes. For example ] links to the German Wikiquote (please note the project prefix should come first).
To link to the "surfeit" Wiktionary entry from Wikipedia, you would include the wikilink markup ] which would appear as wikt:surfeit or the wikilink markup ] which would appear as surfeit.
See the meta-wiki page for available project prefixes and details if interwiki linking. For a complete list of current languages and language codes, see the Wikipedia List of Wikipedias.
In the main namespace, interwikis always link to pages i.e. entries with exactly the same title in the different language versions of Wiktionary, even if that page is a redirect page. This is different from Wikipedia's translation-based method. It is not necessary to add such links to the English Wiktionary by hand as this is done by the internal system. In the Category, Wiktionary, Help, and other namespaces, it is dealt with on Wikidata.
Interwiki links in other namespaces, such as for templates, link pages with similar functionality (not necessarily identical titles). For details about place interwiki links on templates, see Help:Documenting templates.