Definition: hashtag

A topic identifier in a tweet or other social network message that is indexed by the service so that all the user postings with that hashtag can be searched and read. The hashtag is the name of the topic with a number sign (#) prefix, also called a hash mark.

Hashtags are created for anything and everything, including people, businesses, organizations, sports teams, political parties, hobbies, events and philosophies, as well as rants and raves. For example, athletes might be interested in #triathlon or #ironman. See hash, tag, Twitter and bashtagging.

The Word Has Become Everyday Vocabulary

In this example at the Philadelphia airport, "hashtag worthy" means the trip is worth talking about.