Tagging p. pr. & vb. n. : of Tag

Example:
Why when Tagging photos on Facebook does it start getting slower and slower?

Taxonomy
metadata, Flickr, photos, separators, syntax, free encyclopedia, folksonomy, tagging systems, Web 2.0, free-form, Articles with unsourced statements, computer file, History, collections, tagging system, index page, controlled vocabulary, machine tag.

Defination from the web:
tag is a non-hierarchical keyword or term assigned to a piece of information (such as an internet bookmark, digital image, or computer file) ... In a tagging system, there are an unlimited number of ways to classify an item, and there is no "wrong" choice. ...