There is the US Customary Cup at about 236mL, the US Legal Cup at 240mL, the Metric and Canadian Cups at 250mL, the Imperial Cup at about 284mL, the Latin American Cup (which seems to be any of the other previously mentioned cup sizes, depending on where you are), and the Japanese Cup at 200mL.
> An imperial fluid ounce is 1⁄20 of an imperial pint, 1⁄160 of an imperial gallon or exactly 28.4130625 mL. A US customary fluid ounce is 1⁄16 of a US liquid pint and 1⁄128 of a US liquid gallon or exactly 29.5735295625 mL, making it about 4.08% larger than the imperial fluid ounce.
Worse, the ounce, as a unit of mass, has two different meanings in English law, one of which is so obscure and differs so very little from the internationally agreed quantity, that it is hard to believe that it could ever matter, but strictly if you are thinking about the law of food labelling - and I do sometimes - then you always have a slight twinge at the thought.
However, in contradiction to least one remark about the universality of cups, they are not. British cups are, like US cups, half a pint, but British pints are larger. A British cup is 10 fluid ounces while a US cup is 8 (or thereabouts, I am not sure how standardised it is).
So I have to remember to adjust by a factor of 20% when working from a US recipe in the UK, or just convert to weight.
Where I live now a "cup" is typically 200ml unless I am measuring rice into a rice cooker.