With a pointer to the FAA (thank you, Prof2MDA) as the source of this terminological excess, I did some research (using my trusty US-sourced CD containing lots of FAA bits and pieces). A bit of study is always a good thing as one invariably learns something in the process.
The term, "forward slip" indeed is to be found within some areas of the FAA flight training and flight standards bookwork.
For the interest of others who might, like me, not be working within the US environment and not have come across the term before, may I cite some references which you might care to examine -
(a) AC 61-89D, Appendix 1, Lesson #37
(b) FAA-H-8083-3, Chapter 7, from which comes the graphic referred to by dvt (at Fig 7.6). Interestingly, this text makes it clear that the two are the same, except for the (groundbased runway frame of reference) observed track.
(c) Order 8400.10, Volume 5
(d) FAA-S-8081-5C, -6A, -14
(e) Dictionary of Aeronautical Terms
If I may quote from the last document (even if I might take the view that the definition is somewhat inadequate) -
"slip (aircraft flight maneuver).
A maneuver in which the aircraft moves through the air sideways, rather than straight ahead. Slipping produces a large amount of drag so the airplane can descend at a steep angle without gaining excessive speed. An airplane is slipped by crossing its controls, using right rudder and left aileron.
A forward slip is one in which the aircraft turns its side into the wind, but continues in forward flight.
A slide slip is one in which the aircraft continues to point straight ahead, but slips to the side."
Do I have to eat of the weekly umbles pie as befits my lowly status for not knowing more about this matter ? .... I guess so.
Do I still think that the two are the same thing waffling as distinct entities ? .... absolutely.
Is either view more important than the other in the overall scheme of things ? .... probably not.
So far as structural implications might be concerned, I would be less concerned with moderate slip angles than with rapid and varying rudder inputs. There is, also, the possibility of asymmetric stall and unintended spin if the manoeuvre is done at lowish speed and in a ham-fisted sort of way. As always, we in the field do not have access to the manufacturer's design records ... therefore, if something is not prescribed in the manufacturer's documentation then the manufacturer ought to be consulted prior to embarking on one's own little flight test card.
In sim training, I have seen some prodigiously extreme slip angles employed successfully for the purpose of recovering the aircraft from an all-engines failed scenario.
In real life line operations, though, the intentional use of slip (other than that which necessarily is used during the final stages of a crosswind approach and landing) is both unpleasant and unnerving for the SLF and probably has no routine place in large aircraft operations.
Having said all that ... as a very young and comparatively inexperienced pilot many years ago on glider tug operations using the lovely SuperCub .... nothing since gives me the routine delight as that which I derived from an extreme sideslip down to the flare with the tow rope just clearing the boundary fence by inches .... and occasionally leaving the last few inches on the said boundary fence. Probably not very sensible flying and I don't think that I would have any interest in doing so any more .. but it was all rather good fun at the time ...