My Google Activity Report typically shows access from "Windows", "Linux" and "Android" platforms. This month, it is also showing access from the "X11" platform. What would report as "X11"? A question here simply suggests that "X11" means "Linux". However, "Linux" has previously (and still) shows up as a seperate entry.
X11 is version 11 of the X Window System.
And the X Window System is that piece of software which talks to your video hardware and displays bitmapped graphics on your monitor. It underlies your UNIX/Linux desktop environment (e.g. GNOME, KDE, Xfce, etc.) and graphical applications, all of which talk to X at a very low level in order to display graphics on the screen.
As to why Google might identify you as "X11" instead of "Linux" that one is also fairly easy. Most web browsers, in their User-Agent strings, identify the operating system on which they're running, such as Windows, Mac OS X or Linux. But certain versions of Mozilla-based browsers (e.g. Firefox) list the operating system in the User-Agent string as "X11".
For instance, an example Iceweasel User-Agent string looks like:
If you used a browser where "Linux" was missing from the User-Agent string, or used a UNIX operating system which wasn't Linux, then Google might not have identified your operating system correctly and fallen back on the generic "X11".