A T-bill is a short-term loan you make to the United States government. The government needs money now, so it sells you a piece of paper (today, an electronic record) and promises to pay you back a fixed amount on a set date — usually in four weeks, three months, six months, or a year. You buy it for less than that final amount, and the difference is what you earn. So a T-bill does not pay interest along the way the way a bank account does; you make your money at the end. 'T-bill' is the short, everyday name for a Treasury bill, and it is what people in finance and the news actually say. Because the US government is seen as the borrower least likely to fail, T-bills are treated as the safest place in the world to park money for a short time, and their rate is used as the 'risk-free rate' that everything else is compared to. This is a business-news word, not a general one. Note also that Treasury *bills* are the short ones — the longer versions are Treasury notes (T-notes, 2-10 years) and Treasury bonds (T-bonds, 20-30 years).