A highlight (commonly abbreviated as HL, sometimes called a mention or ping) is a way to draw someone's attention to the chat if they are online. By default, a highlight is triggered when a player's username is written out in the chat. The list of what words get highlighted may be customised in osu!'s in-game chat options.
It is possible to ignore highlights from specific users by adding their usernames to the ignore list with
@h appended to them.
An example of a highlight notification
When a chat message triggers highlight, the sender's username on that line becomes green, and the message is added to a
#highlight chat tab. Additionally, if the osu! window is inactive, its icon will flash on the taskbar.