or Repurchase Agreement: A transaction in which one party sells a security to another party while agreeing to repurchase it from the counterparty at some date in the future, at an agreed price. Repos allow traders to short-sell securities and allow the owners of securities to earn added income by lending the securities they own. Through this operation the counterparty is effectively a borrower of funds to finance further. The rate of interest used is known as the repo rate.
Source: Commodity Futures Trading Commission