History lesson time:
In the 1960's, a bunch of people working at GE, MIT, and AT&T's Bell Labs made an operating system for mainframes. The way mainframes worked, you'd have a gigantic computer that took up a whole room and then you'd have a whole bunch of terminals that could take turns running things on the computer. They called this operating system Multics, which stood for Multiplexed Information and Computing Service.
Making a system like this is really complicated, so a few years later, Dennis Ritche (who just passed away last week), Ken Thompson, Brian Kerninghan, and a few other people decided to make a simpler operating system. They wanted it on a smaller scale, so they to call it "Unics", since "uni", or one, is simpler than "multi", or many. That name ended up getting changed to "Unix".
Unix hung around for a long time, being licensed by AT&T to groups like the University of California, IBM, and HP. Unix even forms the core of Mac OS X today. Because it was licensed to educational institutions, it was used a lot for educational purposes so a lot of people got exposed to it. One day in 1992, a guy named Linus Torvalds gets on a mailing list and asks for help writing his own, completely free operating system. It was designed to be similar to, and compatible with, Unix so it was called "Linus's Unix", which got shortened to Linux.