hash tag

An economical and searchable means of specifying the subject of a message on Twitter, created by prefixing a word or short hyphenated phrase with a hash symbol (#). Multiple messages labelled with identical hash tags generate trending topics which can be automatically collected and ranked according to their popularity.

Chandler, Daniel––Munday, Rod (2011): A Dictionary of Media and Communication. Oxford University Press. Oxford. p. 182. See the Online Edition