A (usually long) series of wikipedia articles, in which the first article begins the trail and other articles are read by clicking on embedded links in the first or succeeding articles. Trails are followed until the reader's natural curiosity is satisfied, until the reader has run out of time, or until the reader has passed out unconscious.
Some wikitrails become so addictive that the reader obsessively finds him- or herself obsessively clicking on link after link, even though it is well past the reader's bedtime and there will inevitably be hell to pay for this tomorrow. These types of wikitrails are also called wikiholes.
I followed this interesting wikitrail on the languages of Spain and somehow ended up in the French Pyrenees!