Here's a closer look at the year's 10 biggest moments, as determined by Twitter:
Terror in Paris, January and November
The world rallied behind victims of terrorism in Paris more than once this year, and much of that support was expressed using the hashtag #PrayForParis after the November 13 attacks around Paris that killed more than 120 people and #JeSuisCharlie after the January attack on the offices of magazine Charlie Hebdo. Other conversations emerged on social media in reaction to the violence, such as Muslims using the hashtag #NotInMyName in opposition to religious extremism.
"The Dress," February
Is it blue and black? Is it white and gold? Everyone can agree on one thing: An argument on social media earlier this year about the colors people see in a photo of a dress got blown way out of proportion.
Caitlyn Jenner, April
2015 was a huge year of firsts for Caitlyn Jenner. As she introduced herself to the world, Jenner set the world record for reaching 1 million followers on Twitter faster than anyone before her.
I'm so happy after such a long struggle to be living my true self. Welcome to the world Caitlyn. Can't wait for you to get to know her/me.— Caitlyn Jenner (@Caitlyn_Jenner) June 1, 2015
Same-sex marriage, June
Following the Supreme Court's ruling that same-sex couples have a constitutional right to marry, politicians from both parties weighed in. Supporters of the decision celebrated using the hashtag #LoveWins.
Women's FIFA World Cup, July
Millions of people watched the U.S. team beat Japan in the final match of the Women's FIFA World Cup in July, and the team was showered with love from around the country when they returned home, including a ticker tape parade through New York City. Reports about discrepancies in their pay with winners of the Men's FIFA World Cup started a discussion about equal pay in professional sports.
Pluto fly-by, July
Everyone's favorite former planet was ready for its close-up in July, when NASA's New Horizons probe flew closer to Pluto than any space-craft before it, sending back incredible new images to Earth.
School mistakes homemade clock for bomb, September
A 14-year-old student named Ahmed Mohamed was arrested after he brought a clock he made into school in September and school officials thought it was a bomb. The #IStandWithAhmed hashtag became a way to support the teen and express concerns about racial profiling.
GOT MY CLOCK BACK FINALLY!!😆😆 pic.twitter.com/7k9GXB7M4H— Ahmed Mohamed (@IStandWithAhmed) October 23, 2015
Black Lives Matter, year-round
Although the #BlackLivesMatter hashtag originates well before 2015, Twitter said it played an important role in several conversations this year, including the murder of nine people attending a bible study in Charleston and the protests in Baltimore in reaction to the death of Freddie Gray.
Refugee crisis, year-round
As hundreds of thousands of people flee from violence in the Middle East, places like Europe and the United States continue to discuss their role in helping the refugees. Pope Francis was among those urging anyone who is able to take in refugees using the popular hashtag #refugeeswelcome.
International elections, year-round
National elections took place in Argentina, Canada, Singapore, India and the United Kingdom this year. A map demonstrating how many people were using the hashtag #BBCDebate when it aired in April showed just how many people were talking about the UK elections.
What do you think was the most talked about moment of the year? Share your thoughts in the comments.