What geometric character forms the Slack logo’s central symbol?

An octothorpe forms the central geometric character of the Slack logo.

The octothorpe, commonly called a hash sign or number sign, is the four-armed character associated with Slack’s original logo. Slack’s visual identity surrounded and reshaped that central form with four colored elements, producing a distinctive symbol for the workplace messaging service.

Slack launched publicly in 2013 after evolving from an internal communication tool created during development of the game Glitch. The logo’s four-part construction was intended to feel organized and modular, echoing the way conversations and channels are arranged in the product.

Many people simply call the symbol a hashtag, especially because the same character is used for social-media tags. “Octothorpe” is the more specific typographic name for the character itself; a hashtag usually refers to the character when it precedes a searchable word or phrase.