Can MacOS Dictionary be invoked by a URI?
Can MacOS Dictionary be invoked by a URI?

Yes: dict://
Example HTML code: &lt;a href=&quot;dict://foobar&quot;&gt;Lookup 'foobar' in Dictionary&lt;/a&gt;
Tested with Safari under macOS Catalina using:
&lt;!DOCTYPE html&gt;
&lt;html&gt;
  &lt;head&gt;
    &lt;meta http-equiv=&quot;content-type&quot; content=&quot;text/html; charset=UTF-8&quot;&gt;
    &lt;title&gt;Dictionary Lookup&lt;/title&gt;
  &lt;/head&gt;
  &lt;body&gt;
    &lt;p&gt;&lt;a href=&quot;dict://foobar&quot;&gt;Lookup 'foobar' in Dictionary&lt;/a&gt;&lt;/p&gt;
  &lt;/body&gt;
&lt;/html&gt;

When clicking the hyperlink, one is presented with:



I am wondering whether there is a URI syntax that will look it up in a specific dictionary. (And what is that syntax.)

If there is, I do not know it and I tried modifying dict:// adding the different tabs in the Dictionary app and none worked.
The only thing that worked was dict:// and dict:// followed by the search word, e.g. dict://foobar and in the case of the later syntax it open to the last active tab of the app. For example, if Wikipedia was the active tab when the app was closed it opens e.g. foobar there.