In my opinion, the use of the phrase 'forward slip' only serves to cause confusion.
I agree, and appreciate Lomcevak's good review of the definition.
In my opinion. an airplane flies through air, either straight, or a little sideways. If it's deliberately sideways, it's a slip (or skid). If the air mass, through which the plane is flying is moving relative to the ground, that's a different thing, and doesn't change what the pilot is controlling the airplane to do - it's either flying forward, or being slipped.(flying a little sideways), so just call it a sideslip, and then be assured that the pilot is still flying the plane relative to the surface also and trust that they will compensate for that too!