ORIGINAL: HarryC
I am not aware of any fundamental difference and have never heard the term ''forward slip'' in the 30 years I have been full-size flying! It is always sideslip, whether to approach in cross wind or to lose height rapidly because you have messed up a glide/engine failure approach! One requires more power to prevent that rapid loss of height, in the other situation you actually want that rpaid loss, but that apart they are the same!
H
A forward slip and side are essentially the same thing but at least in Air force flight training they teach us that they are different. I think that it is mainly to teach us that even though they are the same control inputs they can be used for different purposes, ie a crosswind landing or rapid altitude loss. I am suprised you have never heard the term forward slip. I was taught the term forward slip the moment I entered flight training on the civilian side and they use both terms in military flight training as well.