Twitter symbol

• String starting with "#"
• Modern categorizing aid
• Link to a theme involves witch’s crossing close to midnight
• Symbol of modern communication
• Character in many a tweet
• It shares a key with a "3"
• Word or phrase to aid online searches — hag's hat (anag)
• Content-sharing convenience

Wiktionary Word definitions in Wiktionary
n. 1 (context Internet English) A metadata tag, signaled by a preceding hash sign, used to label content. 2 (context informal English) The hash sign itself. vb. (context transitive Internet English) To label (a message) with a hashtag.

Wikipedia Word definitions in Wikipedia
A hashtag is a type of label or metadata tag used on social network and microblogging services which makes it easier for users to find messages with a specific theme or content. Users create and use hashtags by placing the hash character (or pound sign) ...