And of course, I'm one of those old-fashioned types who use the term "backstitch" for the process that most know as "understitch" (I've also heard it called "pinstitch" and "edgestitch on the facing").
My sewing terminology comes from a combination of my childhood of 1960's Enid Gildchrist magazines, my tutors at RMIT and from working (in the fashion and craft industries) in Australia and a few other countries - with people from all over the world. I've found that although we all use different terminology, we speak the same language when we explain the processes.
So this is what I call edgestitch... which is like backstitching (or understitching or pinstitching...) but on the outside of the garment or bag. Mostly I just call it "topstitching".
The predominant language of sewing in the published world seems to be American....I guess it's a population issue. I've noticed that Australians - who previously would have used Bristish terminology - are now using American terminology. It's a sewing-language I somehow bypassed in my sewing education, but I'm catching up through blog-world and my expanding sewing reference library.
In dressmaking books, I struggle to find a reference to "staystitching" as anything other than holding the shape of a curve on a single layer of fabric. When I speak to fashion-industry friends we all understand it as stitching that holds things in place before a seam is sewn - whether that be a curve on a single layer of fabric or straps and button loops in place. Anytime the stitch length isn't altered, something is held in place and the stitches stay in.... that's staystitch. That is, until you try to find it in a book.
I guess because in mass-production, there's rarely time to tack (or "baste".... a term that always brings turkeys to mind!) anything - so we tend to staystitch rather than mess about with stitch lengths or unpicking. In my language, tacking (or basting) means that the stitches are temporary. 
I also use industry-style nicks as registration marks to bring the cut pieces of the garment together. Nicks can be cut accurately through layers and layers of fabric..... and they are marked with either a slit (like a long "U") or a "T" shape symbol. I've never understood those silly triangle notch marks on commercial patterns. (Why give yourself such a large margin for error when you can be accurate to the millimetre?). But give me a commercial pattern with silly triangles and I'll know what they mean (same thing as nicks, only more difficult to cut!).
...which brings up the whole issue of metric versus imperial. I was brought up in the metric age and I've never been a patch-working girl (who use inches even in countries with metric as the standard). I draft my patterns (in CAD) to be accurate to within two decimal places of a millimetre... but please don't ask me what that is in inches. In recent years, I've had to change my pattern seam allowances to be easily converted to half and quarter-inch (so that the imperial-measurements people can understand them). I'm afraid that's as far as my understanding of fractions of inches go!
There are also conventions and techniques that change with age, geography or simply the school you went to - darts or seams presed to the front, back or open, the way seams are clipped.... that sort of thing. This language we speak has many dialects!
I'm trying to compile a list of terminology here - trying to cover all "dialects". I'm interested to hear if you've heard any that I've missed - for the processes above, below or others not mentioned.
TOILE (Australia, UK and elsewhere) = MUSLIN (USA)
A test-garment, sewn in calico or other cheap fabric.
Duckbill Scissors = Applique Scissors
(Of hems) Fine Double-Turned = Fine Double Neatened = Pinstitched (yep - there it is again, meaning something completely different!) = Rolled
A super-fine hem that is turned twice and machine stitched - usually on a sheer fabric or handkerchief edge.
Block (Australia, UK and elsewhere)= Sloper (USA)
The basic garment shape from which patterns are drafted.
(Of seams - to stop raw edges from fraying) Trimming = Finishing = Neatening
Overlocker (Australia, UK and elsewhere) = Serger (USA)
What sewing language to you speak? What's your experience of the varied dialects...?
