I.O.U. stands for 'I owe you' and is a written acknowledgment of a DEBT. It is commonly used in informal transactions between two parties, such as friends or family members. The term can also be used to refer to four-letter crossword solutions: DEBT, CHIT, NOTE, and SCRIP. A MARKER is a six-letter solution, while VOWELSOFPOVERTY is a fifteen-letter solution. All of these terms refer to an I.O.U. and are used to acknowledge a DEBT.