What is the pattern '' referred to in Drupal speak? Is the pattern  used to denote the front page in Drupal referred to as token or something else? Does it exist in Drupal 8?
According to this Drupal 8 issue queue, &lt;front&gt; is a "special token." Yes, it still works in Drupal 8.

Reading the whole issue queue will give you some insight into the core developers' thought processes as they decided whether or not to support this legacy Drupalism in Drupal 8. They had a lot of good reasons to drop support for it, but they erred on the side of caution and left it in.

You can also try it for yourself in a Drupal 8 site; it definitely still works!

Help text for adding a menu link says:


  Start typing the title of a piece of content to select it. You can also enter an internal path such as /node/add or an external URL such as http://example.com. Enter &lt;front&gt; to link to the front page.


I guess you might want to give more weight to Clive's comments above, since his answer is based on the actual code and comments in Drupal core. Technically, all strings processed by index.php are considered "paths" and the ones that resolve to valid destinations are "routes." (Under the hood, Drupal 8's routing system is based on the routing provided by the Symfony HTTP kernel, and is used to connect paths to controller logic.)