The "#" symbol was always called the "hash character' or "number sign", both names are valid. On sites like twitter if you place it 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. Thus making it easier to find related content. You "tag" using "the hash character" that's why people named it a hashtag. For more info see here: en.wikipedia.org/wiki/Hashtag

90's kids call it the number symbol, because while the term "hash" for the symbol existed, hashtags didn't, so in the 90's people used it for numbering mainly.