Hashtag

A hashtag is a type of label or metadata tag used on social network and microblogging services which makes it easier for users to find messages with a specific theme or content. Users create and use hashtags by placing the hash character # (also known as the number sign or pound sign) in front of a word or unspaced phrase, either in the main text of a message or at the end. Searching for that hashtag will yield each message that has been tagged with it. A hashtag archive is consequently collected into a single stream under the same hashtag. For example, on the photo-sharing service Instagram, the hashtag #bluesky allows users to find all the posts that have been tagged using that hashtag.

Because of its widespread use, hashtag was added to the Oxford English Dictionary in June 2014. The term hashtag can also refer to the hash symbol itself when used in the context of a hashtag.

The pound sign or hash symbol was often used in information technology to highlight a special meaning. (It should be noted that the words "Pound Sign" in the UK refer specifically to currency "£" - extended ASCII character 156 - and not weight.) In 1970 for example, the number sign was used to denote immediate address mode in the assembly language of the PDP-11 when placed next to a symbol or a number. In 1978, Brian Kernighan and Dennis Ritchie used # in the C programming language for special keywords that had to be processed first by the C preprocessor. Since before the invention of the hashtag, the pound sign has been called the "hash symbol" in some countries outside of North America.

The pound sign appeared and was used by people within IRC networks to label groups and topics. Channels or topics that are available across an entire IRC network are prefixed with a hash symbol # (as opposed to those local to a server, which use an ampersand '&').

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