It used to be referred to by many as the number, or pound, sign. Nowadays, thanks to its widespread use on social media, the symbol # is undeniably known as the hashtag.
This function was invented 10 years ago, when technology specialist Chris Messina proposed to use it to create groups on Twitter on August 23, 2007:
It took nearly two years for Twitter to institutionalize the practice, by adding a hyperlink to all hashtags in tweets, on July 2, 2009.
Previously existing Twitter practices, such as Follow Fridays, were turned into one word preceded by a hashtag. More tags were created for other days of the week, such as the popular #MotivationMonday.
In 2010, Twitter added Trending Topics on its front page, through which users could easily discover all hashtags rapidly gaining in popularity.
Another hashtag breakthrough came with the photo-sharing application Instagram, launched in October 2010. "It became quickly clear that one couldn't find pictures again without descriptions," Chris Messina told German press agency dpa.
The term "hashtag" entered the Oxford English Dictionary in 2013, which also named it Word of the Year; it was added to Merriam-Webster's Dictionary in 2014.
Hashtags have since been adopted on all social media platforms, including Facebook, Snapchat and Google+.
The rise of hashtag activism
During the 2009-2010 Iranian election protests, social media users widely used the symbol in their posts, turning the practice into an international style of writing.
Other political protest campaigns of the early 2010s were organized through hashtags, such as #OccupyWallStreet and #Jan25 for the Egyptian revolution which began on January 25, 2011.
The hashtag became a useful instrument to create awareness for social causes and movements.
Over six million people posted a tweet with the hashtag #BringBackOurGirls in support of the Nigerian schoolgirls kidnapped by the Islamist group Boko Haram. Former US First Lady Michelle Obama was among the users who did:
However, "hashtag activism" became criticized by some as a lazy way of taking action, just like signing an online petition. On the other hand, the case of the Nigerian schoolgirls first garnered international attention after supporters massively demanded their safe return through Twitter.
Beyond the numerous "slacktivist" trending hashtags, the popular #BlackLivesMatter demonstrates how a hashtag can also durably unify a community to establish a political movement.
In other cases, hashtags have actually managed to get people who aren't typically activists to leave their computers screens for a good cause.
During the summer of 2014, the hashtag #IceBucketChallenge was an awareness campaign which incited countless people to dump a bucket of icy water over their heads - a fundraising stunt that allowed the ALS Association to raise millions of dollars for research for Lou Gehrig's disease.
#Sarcasm and live-tweets
Beyond social causes and other unifying slogans that spontaneously appeared after terrorist attacks à la #JeSuisCharlie, hashtags are also widely used to express sarcasm, as a form of humorous meta-commentary.
This sometimes excessive approach was parodied in a hilarious sketch by Jimmy Fallon and Justin Timberlake in September 2013.
Beyond the now classic #LOL, Trump's cryptic #covfefe remains popular weeks after he posted a tweet with the typo.
In some cases, the sarcasm becomes brutal - derogatory slogans can be turned into hashtags used to troll individuals or organizations.
Most often, however, hashtags create a sense of community, allowing people with common interests to engage with each other through social media. Late-night show hosts in the US regularly prompt new hashtags that turn into trending topics.
From the Oscars to the Eurovision Song Contest, TV-related live-tweeting is an extremely popular hobby. Those who wish to avoid spoilers should definitely avoid Twitter.
That's especially true if you're behind on the series "The Walking Dead." It was the most popular show on social media for the 2016-2017 season, with two million interactions on Facebook and Twitter for each new episode aired on AMC.
The hashtag's history is directly connected to the development of Twitter, which celebrated it's 10 anniversary last year. Here's a gallery with 10 unforgettable Twitter moments: