A link between two pieces of information. At the time, revolutionary. Obsidian brought that principle to the masses with wikilinks
wiki's with wikilinks brought it to more people than Obsidian does now. Wikilinks is not a very recent thing. 1990s wiki (Ward's e.g) had CamelCaseLinking, WikiPedia (2002) had wikilinks from early on, they were introduced in the UseModWiki code at the start of 2001.