Short for Social Security Number — the nine-digit number the US government gives a person, written like 123-45-6789. You say it letter by letter: S-S-N. It started as a number for tracking what you paid in for your retirement money, but over the years it turned into the number that proves who you are in America. You need it to work, to pay tax, to open a bank account, to rent a home, often to see a doctor. Because it does so much, Americans treat it as a secret: if a stranger gets your SSN they can borrow money in your name. So a real employer will ask for it, but an email or a phone call asking for it is almost always someone trying to cheat you. In everyday speech people often just say "your social" — "Can I get the last four of your social?" — meaning the last four numbers, which is a common way to check identity without giving the whole thing away. Taiwan's 身分證字號 does a similar job, but there is one big difference: in Taiwan you show your ID card as a matter of course, while in America saying the number out loud in public makes people nervous.