On Thursday, popular workplace chat app Slack made its highly anticipated debut on Wall Street that valued the company just under $20 billion.

Slack, which is trading under the stock ticker “WORK,” opted for a direct listing, an uncommon approach allows the company to bypass the traditional public offering process and sell shares directly to the public. Shares were trading up some 50% from its reference price of $26 per share Thursday.

Read More: The amazing life of Stewart Butterfield, the CEO leading Slack to a potential $15.7 billion valuation when it goes public today

Slack’s workplace chat app skyrocketed in popularity since it was created by CEO Stewart Butterfield in 2009. But not many people know the actual meaning behind the company’s name, Slack.

Some people guessed it's supposed to stand for the term used in project management, which refers to the "amount of time a delay could take from a task without causing subsequent problems."

But according to Butterfield, Slack is actually an acronym. It stands for: "Searchable Log of All Conversation and Knowledge."

That's exactly what Slack is, although most people use it for chatting and sharing links and files, and don't worry about the searchability so much - though, as the New York Times reported this week, sometimes people search for their own name in a company's Slack when they take a new job, and don't like what they find.

Butterfield shared this through a Twitter conversation he had with someone who asked about the name's origin, way back in 2016. He also notes Slack's original codename used to be "linefeed."

Slack joins at least one other prominent tech company whose name is secretly an acronym: Yahoo, which originally stood for "Yet Another Hierarchical Officious Oracle," a joking reference to its original product, a web directory.

This is an update to a post by Eugene Kim first published in 2016.