Further confused, in the US I've heard of Numnahs as contour pads, as opposed to square pads. Brits, please, is there a diff? My British girlfriend, who has lived stateside for forever, still cracks up when we say helmet vs. hard hat! ;)
I live in the UK and understand and use all the terms mentioned interchangeably. I will say both box and stall; paddock, field, pasture; manege, school, arena... Halter and headcollar I've heard used all the time interchangeably. Didn't even know there was meant to be a difference. Numnah is a term I've only heard used in certain circles - otherwise 'saddlepad' is more common. If you want terms that are genuinely confusing across continents, try "hack" and "showing". But in general I think the internet has merged the lingo and I certainly wouldn't have known which of the terms discussed in the article were meant to be British and which American...
And then there is Australia; where horses are kept in stables, turned out in paddocks and ridden with a saddle blanket between saddle and horse...