Tinfoil is the thin, shiny, bendy metal sheet you wrap food in — the roll in your kitchen drawer. Here is the honest part: it has not actually been tin for a long time. Real tin foil was used until about the 1940s, and then aluminium took over because it is cheaper and does not leave a metal taste. The name simply stayed. So 'tinfoil' and 'aluminium foil' mean the same object today, and both are correct; Americans tend to say 'aluminum foil' or just 'foil' on packaging, while 'tinfoil' survives strongly in ordinary speech, especially among older speakers and in Britain. Around the kitchen most people shorten it to 'foil': 'cover it with foil', 'a sheet of foil'. It is uncountable — 'some tinfoil', 'a piece of tinfoil'. One extra thing worth knowing, because it comes up online constantly: 'tinfoil hat' means someone who believes wild conspiracy theories, from the old joke about wearing a metal hat to block government mind-rays. If you read 'that's a bit tinfoil-hat', that is what it means.