Hey, this is a really interesting thread. I love words!
Most of the differing American/British words that I know have already been mentioned by some people... I did read in a Bill Bryson book once that what we call Polyfilla (actually a brand name that gets used kind of generically, like Hoover), Americans call 'spackle'. I like that word better!
We call the underground room either 'cellar' or 'basement' here in the UK, but the way I understand it, 'cellar' is more like a storage room whereas a 'basement' (or sometimes 'semi-basement') is an underground room/apartment where you can actually live (i.e. 'He had a basement flat').
Hehe, I confused a Canadian girl I was chatting with once when I mentioned wellies. She thought I was talking about underwear!
We also have Subway (the sandwich shop) in the UK! (It always amuses me to see their poster advertising 'sub of the week' because I instinctively think of 'sub' as in 'submissive' in bdsm-related terms... lol *wants to try the sub of the week*

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On difference I have noticed between American and British English that no one has mentioned yet... Americans tend to just say 'go...' where a British person would say 'go and...' - for example I'd say 'I'll just go and find her', but I think an American would be more likely to say 'I'll just go find her'... ? :?
Like mos, I've also heard that 'gotten' is actually an 'archaic' English word that has fallen out of use here but it's still grammatically correct to use it in British English. (I heard this from my mum who studied English Lit at uni in the '70s; part of their course included studying Old English and Anglo-Saxon so no doubt that was where she picked up the info.)
Oh, another thing I've noticed is that Americans call cushions 'pillows'. For a British person a pillow is what you rest your head on in bed (so what do Americans call them?

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One thing that confuses me a bit are the various distinctions between trainers/sneakers/pumps/plimsolls.

We say trainers and plimsolls in the UK and pumps is also sometimes used to refer to shoes like plimsolls (although I hardly ever hear it; is it regional?) but 'sneakers' isn't used at all here (afaik). So are sneakers like trainers? And do Americans ever talk about 'trainers' or 'plimsolls'? :?
A word for money that I can think of that's used here is 'spondoolies'. This was absolutely new to me until my boss sprang it on me a couple of months ago!

I was like 'What?' and he was like 'It's a slang word for money'... and when I asked around the office everyone else had heard of it except me!
