A contract in which the seller of securities, such as Treasury Bills, agrees to buy them back at a specified time and price. also called repurchase agreement or buyback. Or we can say that a form of short-term borrowing for dealers in government securities. The dealer sells the government securities to investors, usually on an overnight basis, and buys them back the following day.
For the party selling the security (and agreeing to repurchase it in the future) it is a repo; for the party on the other end of the transaction, (buying the security and agreeing to sell in the future) it is a reverse repurchase agreement.