U.S. Treasury Bill
Short-term U.S. government bearer securities with maturities of three, six and twelve months. They are issued on a regular basis and have a dominating position on money markets. They are issued at a discount to par value and the purchaser receives the full face value at maturity. Nearly one-third of marketable U.S. Treasury debt is concentrated in Treasury bills. The purchase and sale of such bills through open market operations forms a key part of U.S. monetary policy.
See also: Bearer Shares/Bonds, U.S. Treasury Note, U.S. Treasury Bond, http://www.treasurydirect.gov/indiv/products/prod_tbills_glance.htm