Hashtags are words or phrases prefixed with the symbol #, a form of metadata tag. Short messages on microblogging social networking services such as Twitter, identi.ca or Google+may be tagged by including one or more with multiple words concatenated, such as those in:

Wikipedia is an #encyclopedia

Then, a person can search for the string #Wikipedia and this tagged word will appear in the search engine results. Such tags are case-insensitive, with CamelCase often used for readability.

Hashtags are mostly used as unmoderated ad-hoc discussion forums; any combination of characters led by a pound sign is a hashtag, and any hashtag, if promoted by enough individuals, can “trend” and attract more individual users to discussion using the hashtag. Hashtags are neither registered nor controlled by any one user or group of users, and neither can they be “retired” from public usage, meaning that hashtags can be used in theoretical perpetuity depending upon the longevity of the word or set of characters in a written language. They also do not contain any set definitions, meaning that a single hashtag can be used for any number of purposes as espoused by those who make use of them.

Because of their loose nature, hashtags often become more recognized as associated with particular topics of discussion based upon a more-specific spelling of the hashtag (i.e., “#cake” as opposed to “#thecakeisalie”) that will be differentiated from a more-general spelling.

Hashtags also function as beacons in order for users to find and “follow” (subscribe) or “list” (organize into public contact lists) other users of similar interest.

The feature has been added to other, non-short-message-oriented services, such as the user comment systems on YouTube and Gawker Media; in the case of the latter, hashtags for blog comments and directly-submitted comments are used to maintain a more constant rate of user activity even when paid employees are not logged into the website.[6][7] Real-time search aggregators such as the former Google Real-Time Search also support hashtags in syndicated posts, meaning that hashtags inserted into Twitter posts can be hyperlinked to incoming posts falling under that same hashtag; this has further enabled a view of the “river” of Twitter posts which can result from search terms or hashtags.
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