A hashtag is a word or phrase preceded by the pound sign (#) that helps organize and label content on social media and other online platforms. When you add #science to a post about your volcano experiment, you're creating a clickable link that connects your post to everyone else's posts about science.

Hashtags work like filing labels for the internet. Imagine if every book in a library had sticky notes indicating its topic: you could find all books about dinosaurs just by looking for the “dinosaurs” note. That's what hashtags do for posts online. Click on #basketball and you'll see posts from people all over the world discussing basketball.

The term comes from combining “hash” (an old name for the # symbol) with “tag” (a label). Twitter popularized hashtags in 2007, and they spread to Instagram, TikTok, and other platforms. People use them seriously (#ClimateChange) and playfully (#MondayMood). Some hashtags become so popular they trend, meaning thousands of people are using them at once.

You might see hashtags used sarcastically too, like someone writing “I love doing homework #not” to show they mean the opposite. While hashtags started online, people now sometimes say them out loud as a joke: “That test was hard, hashtag: nervous!”