(on social-networking Web sites) a word
or phrase preceded by a hash mark (#), used within a message to identify a keyword or topic of interest and facilitate a search for it: The hashtag #sandiegofire was used to help coordinate an emergency response to the fire.
a hash mark used in this way.
verb (used with object), verb (used without object), hash·tagged, hash·tag·ging. 3.
to add a hashtag to (a word, topic, or message): Someone on Twitter just hashtagged the film festival.