What does the C stand for in USB-C?

According standards for USB there are USB-A, USB-B, and USB-C. Different speed and/or plug standards. USB-C uses a mini-connector and is capable of handling up to 5 Gb/s transfer speeds.

It's just the letter that was next after the letter, "B." If they come up with a new USB standard, it would likely be called, "D."