Visqueen is polythene, as already stated, Terram is and industrial fabric for road building etc. very expensive, any merchant or garden centre will sell weed suppression fabric which is what you want, and very cheap too. Works well.
Derek.
I asked because where I bought my decking doesn't seem to have any and I didn't know what it is called. I'll nip along to B&Q as that stuff looks like just what I need.
There's various qualities of weed suppressant fabric - for use under decking you can probably get away with the cheapest, as it's not going to be disturbed at all.