Dollar Roll
A special type of repurchase agreement in which the security, transferred to the investor as collateral, is a mortgage-backed security. The investor who sells the security gives up the cash flows during the roll period, but has use of the proceeds.

Notes:
It's similar to a reverse repurchase agreement.

An agreement with a commitment by the seller (dealer) to buy a security back from the purchaser (customer) at a specified price at a designated future date. Also called a repo, it represents a collateralized short-term loan for which, where the collateral may be a Treasury security, money market instrument, federal agency security, or mortgage-backed security. From the purchaser's (customer's) perspective, the deal is reported as a reverse repo.