hashtag

noun

• a word or phrase preceded by a hash sign (#), used on social media sites such as Twitter to identify messages on a specific topic:spammers often broadcast tweets with popular hashtags even if the tweet has nothing to do with them the hashtag #riotcleanup is being used by community members to coordinate some post-riot street-cleaning
• the hash sign (#):Hunt mistook an @ for a hashtag while tweeting derogatory thoughts about him