@reply
Alternatively called a tag or mention, @reply is a function used on most social networks to reply to someone, or reference another user. This shortcut is done by including the at symbol (@) followed by a username. For example, including @computerhope in your tweet on Twitter would send computerhope a notification, and automatically create a hyperlink to the computerhope profile for others to click.
Because it is used to reference users on social networks, the @ symbol has become a common abbreviation for a handle or username online. For example, if someone asks, "what's your at?" when discussing Instagram, they most likely want your username.
Because this feature is so commonplace, many platforms display a mini search bar when you type the @ symbol, allowing you to quickly select accounts you interact with on the platform.