ES6 refers to ECMAScript version 6, which was the official name of the latest version up until around December 2014. I believe ECMAScript 2015 was coined by Dominic Denicola to put additional pressure on TC39 to publish that year.
You are right, ES6 has a lot more traction than I thought. The reason I posted that was because I had a lot of trouble understanding what ES6 was referring to when I first started researching it, and it seemed like an amorphous standard with varying features depending on who you ask. But no, it's an actual standard published as "version 6".
Incidentally I misspelled "ECMA" in my original comment, so now it's easier to see why it won't catch on anytime soon.
