A hashtag is a metadata tag that is prefaced by the hash (also known as pound or octothorpe) sign, #. On social media, hashtags are used on microblogging and photo-sharing services such as Twitter or Instagram as a form of user-generated tagging that enables cross-referencing of content by topic or theme. For example, a search within Instagram for the hashtag #bluesky returns all posts that have been tagged with that term. After the initial hash symbol, a hashtag may include letters, numerals, or underscores.