The pound sign is the hashtag.
An example of a hashtag is to write about the president on Twitter and end your tweet with #Obama.
Origin of hashtagfrom hash + tag
Origin of hashtaghash 3 tag 1
(third-person singular simple present hashtags, present participle hashtagging, simple past and past participle hashtagged)
- (Internet) To label (a message) with a hashtag.
hashtag - Computer Definition
A means of providing a common topic identifier in text and chat messages so they can be searched as a group. Commonly used in tweets, the hashtag uses a number sign (#) prefix followed by text. Hashtags can be created and used to identify anything, including people, businesses, organizations, sports teams, political parties, hobbies, events, philosophies, moods, rants and raves. For example, the hashtag for the United States Marines is #marines. The athletically inclined might search for #triathlon or #ironman. See Twitter and bashtagging.