Machine Tags

From Wikipedia:

Machine tags are a form of tag, introduced in January 2007 by Flickr. The format was first devised for geolicious in November 2004, to map del.icio.us bookmarks, and gained steam through adoption by mappr and Geo Bloggers to map flickr photos.

They comprise three parts, a namespace, a predicate and a value. For example, "geo:long=123.456" is tag for the geographical longitude coordinate whose value is "123.456".

In February 2007 machinetags.org was setup to try and standardize machine tagging conventions and promote their use.

Machine Tag History / More Reading

24 January: Flickr 'officially' introduces machine tags

Meta Tags: The Poor Man’s RDF? - discusses emergence of machine tags on del.icio.us, e.g. the "for:" tag was in use before it became formalized.