What does istioctl x command refers to?

I see in the official documentation references to commands starting with istioctl x ... but there is not official definition for the usage of x.

As far as I can infer from the istioctl manual, it appears to be a shorthand for experimental.

The command istioctl | grep x does not show any reference to the x usage neither.

Solution

You are correct - x is shorthand for experimental.

You can see more information if you issue istioctl x without any additional commands:

$ istioctl x
Experimental commands that may be modified or deprecated

Usage:
  istioctl experimental [command]

Aliases:
  experimental, x, exp
...