Source : Webster's Revised Unabridged Dictionary (1913)

Greenback \Green"back"\, n.
   One of the legal tender notes of the United States; -- first
   issued in 1862, and having the devices on the back printed
   with green ink, to prevent alterations and counterfeits.

Source : WordNet®

greenback
     n : a piece of paper money (especially one issued by a central
         bank); "he peeled off five one-thousand-zloty notes"
         [syn: {bill}, {note}, {government note}, {bank bill}, {banker's
         bill}, {bank note}, {banknote}, {Federal Reserve note}]