Most links on Wikipedia serve to connect one piece of encyclopedic content to another. However, from time to time we use links specifically and exclusively to define a difficult/unusual/non-English term. In these cases, the reader is not taken to an encyclopedia article, but to a definition -- perhaps a Wiktionary or Wikispecies entry, or perhaps a Wikipedia article that would be better placed on one of these sister projects. While these links are often indispensable, they can also be frustrating, since they violate the principle of least astonishment by taking the user to a bare definition rather than an encyclopedia entry.
It would be constructive to clearly distinguish between links to articles and links to definitions, just as we distinguish between links to articles that exist and links to articles that do not exist. It is also useful to make definitional links somewhat less obtrusive than encyclopedic links. In most skins and browsers, this can be accomplished efficiently by the use of the HTML "span" element, specifically <span class="nounderlines">. However, it would be preferable if this could be somehow hard-coded into Mediawiki.