The @-mention convention lets a Twitter or X user mention another account by placing an @ symbol before its username.
The practice began with users rather than Twitter’s original product design. In August 2006, user Chris Messina proposed using the @ symbol to address people within messages, borrowing a convention familiar from internet chat and discussion systems. Other users adopted it, and Twitter later built the behavior into the service.
An @-mention is different from a hashtag. A mention identifies an account, while a hashtag labels a topic or phrase. It is also different from a reply: a reply is a post composed in response to another post, although replies commonly contain mentions.
The convention became important as Twitter developed from a simple SMS status service into a public conversation network. Usernames can change, but the @ symbol remains the recognizable marker for directing attention to an account.