The symbol # is most commonly known as a number sign, hash, or pound sign. Other names include octothorpe and hashtag. The symbol is used for a variety of purposes, including the designation of a number (for example, “#1” represents “number one”), an abbreviation for pounds avoirdupois, a metadata tag on social media platforms, and a button on touch-tone telephones. The symbol is defined in Unicode and ASCII as U+0023 # NUMBER SIGN (HTML #).
The musical symbol called sharp (♯) is different and has a different Unicode assignment. A sharp cannot have horizontal lines (to avoid being obscured by the horizontal musical staff lines), while in most fonts the number sign has horizontal lines.
Hashtag, hashtag symbol
Twitter documentation refers to it as “the hashtag symbol” The word “hashtag” is often used when reading social media messages aloud. For instance the text “#xyz” is often read out loud as “hashtag, x, y, z” (as opposed to “hash, x, y, z” or “number sign, x, y, z”).
hashtag – a word or phrase preceded by a hash sign (#), used on social media sites such as Twitter to identify messages on a specific topic.
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 (or pound sign) # (also known as number sign or octothorpe) 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 then present each message that has been tagged with it. A hashtag archive is consequently collected into a single stream under the same hashtag
Using hashtags on Twitter
A hashtag—written with a # symbol—is used to index keywords or topics on Twitter. This function was created on Twitter, and allows people to easily follow topics they are interested in.
Using hashtags to categorize Tweets by keyword:
People use the hashtag symbol (#) before a relevant keyword or phrase in their Tweet to categorize those Tweets and help them show more easily in Twitter search.
Clicking or tapping on a hashtagged word in any message shows you other Tweets that include that hashtag.
Hashtags can be included anywhere in a Tweet.
Hashtagged words that become very popular are often Trending Topics.
Tips for using hashtags:
You cannot add spaces or punctuation in a hashtag, or it will not work properly.
If you Tweet with a hashtag on a public account, anyone who does a search for that hashtag may find your Tweet.
We recommend using no more than 2 hashtags per Tweet as best practice, but you may use as many hashtags in a Tweet as you like.
Type a hashtagged keyword in the search bar to discover content and accounts based on your interests.
Facebook also graciously offered these 3 usage tips for the hashtag:
Search for a specific hashtag from your search bar.
Click on hashtags that originate on other services, such as Instagram.
Compose posts directly from the hashtag feed and search results.