Heli1 has almost got it right... the X2 is almost a compound helicopter because the X2 lacks a wing surface.
"COMPOUND HELICOPTER- rotorcraft with driven rotors, additional means of propulsion, and using aerodynamic lift." (from VTOL by Mike Rogers 1989).
The words aerodynamic lift in this definition refers to a fixed wing.
The X2 is not a "pure helicopter" but it is also not exactly a compound because it lacks a wing.
slowrotor