Seriously, stop calling every instance of the ‘#’ character a “hashtag“. Sure, call it a hashtag when it’s actually used as one, but don’t apply the term to each and every use of the character. It’s really #annoying!
Applying the term “hashtag” to the ‘#’ character followed by a word was first done by Stowe Boyd in a 2007 blog post. The concept of using hashtags predates the term by a few days and was conceived by Chris Messina in his famous Tweet:
The idea came from IRC, a chat system which predates the World Wide Web, where public channels are prefixed with the ‘#’ character (for example, the channels on freenode). These channels are almost always named after the main topic of the chat (like #archlinux).
On Twitter, hashtags are used to add keyword tags to a Tweet, much like the tags on the bottom of this post. Hashtags are often used to promote a certain event or idea, and popular hashtags may make it to Twitter’s trending list. What’s more, Twitter can aggregate recent Tweets that are tagged with a given #hashtag, making it easier to view tagged Tweets. Hashtags were a convention at first, but they became so popular that Twitter and other social networks made it official.
However, anything that isn’t a ‘#’ followed by a keyword used for tagging should not be called a hashtag; they have their own proper names.
‘#‘ is used in a hashtag, but it’s not a hashtag by itself. This character is called a “number sign” when used before a number. It is otherwise called a “pound sign” in North America and a “hash” everywhere else (whence “hashtag”). It also has other names, none of which is “hashtag”.
‘♯’ looks similar to ‘#’ but is completely different. This is the sharp sign, which raises a musical note up a half step. People often use ‘#’ in its place because it’s easier to type (including Microsoft in its C#, F# and J# languages), but there’s a key difference: ‘#’ has horizontal lines with slants, and ‘♯’ has vertical lines with slants.
Go ahead and start using ♯sharptags if you want, but don’t ever call ‘#’ a “hashtag”.