A 'tag' is a keyword or term assigned to a piece of information such as an internet bookmark, digital image, or computer file. This kind of meta data helps describe an item and allows it to be found again by browsing or searching. Tags are chosen informally and personally by the item's creator or by its viewer, depending on the system. On a website in which many users tag many items, this collection of tags becomes a folksonomy.

Tagging was popularized by websites associated with Web 2.0 and is an important feature of many Web 2.0 services. It is now also part of certain desktop softwares.