The symbol # is a “hash”. The “tag” Is the word that follows.

The hash exists as a signal to a computer program indexing posts by topic that the following characters before the next whitespace are meant to signify a tagging of a topic. Twitter started doing this a long time ago, to give users a way to categorize their posts by topic without needing a separate interface. (Did you know that the original 140 character tweet length was so it could fit in a single SMS message? Twitter used to be operable over text message). It’s likely that the # symbol was chosen because in URLs it’s used to signify a page anchor in HTML documents, such as a specific header or paragraph, and so similarly here it’s used to find specific topics; you’re anchoring your interest to something specific.

So it’s not a “hashtag” without the hash and the tag. #TIL