Slack is a proprietary business communication platform developed by American software company Slack Technologies. 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 in August 2013.
In March 2015, Slack announced it had been hacked over four days in February 2015, and that some data associated with user accounts had been compromised, including email addresses, usernames, hashed passwords, phone numbers, and Skype IDs. In response to the attacks, Slack added two-factor authentication to its service.
Slack was previously compatible with non-proprietary Internet Relay Chat (IRC) and XMPP messaging protocols, but the company closed the corresponding gateways in May 2018.
"Searchable Log of All Conversation and Knowledge" is a backronym for "Slack".
On April 26, 2019, Slack went public without an IPO and saw its shares soar to $21 billion valuation.