Search engines traffic to Wikipedia
Clickstream event: A user came from a search engine page to a Wikipedia article page.
Empty referer traffic to Wikipedia
Clickstream event: An empty referer to a Wikipedia article page. This could be the result of a browser homepage setting, a security measure, a bot, etc. (source)
Other Wikimedia traffic to Wikipedia
Clickstream event: A user came from some other Wikimedia project page (including other Wikipedias) to a Wikipedia article page.
External websites traffic to Wikipedia
Clickstream event: A user came from an external website to a Wikipedia article page.
Unknown traffic to Wikipedia
Clickstream event: An unknown referer to a Wikipedia article page. This is a catch-all external traffic category, for external traffic that does not fit into any of the other traffic types listed here.
Links traffic to Wikipedia
Clickstream event: A user came from a Wikipedia article A to a Wikipedia article B, where A has a link to B.
Internal search traffic to Wikipedia
Clickstream event: A user came from a Wikipedia article A to a Wikipedia article B, where A does not have a link to B. This happens when the user gets to the second article by searching from the first article, or if the referer is spoofed (according to the Wikipedia clickstream datasets format description).
To keep things simple, we're going with the most likely explanation here and assuming that these clickstream events are internal searches.