A form of United States currency, specifically a Federal Reserve Note, which is the paper currency presently used by the United States. The
ink on the reverse side of Federal Reserve Notes being
green is what led to the nick name "greenback". To a lesser extent, Silver Certificates and United States Notes also had green backs, but the seal and serial numbers on the front were printed in
blue instead of
green ink. They were last issued in 1957 and 1966, respectively. Your parents probably remember them.