Slack is a proprietary business communication platform developed by American software company Slack Technologies and now owned by Salesforce.

Slack offers many IRC-style features, including persistent chat rooms (channels) organized by topic, private groups, and direct messaging.

Slack began as an internal tool for Stewart Butterfield's company Tiny Speck during the development of Glitch, an online game.

Slack launched to the public in August 2013.

According to Butterfield, "Slack" is an acronym standing for "Searchable  Log of All Conversation and Knowledge," which he chose in 2012 to  replace the previous codename, "linefeed."

On January 1, 2021, Slack and Salesforce announced an agreement for Salesforce to acquire Slack for a value of approximately $27.7 billion.

The acquisition closed on July 21, 2021.