I O U

n. signed note acknowledging a debt

IOU is a 3-letter acronym that can mean:A note and acknowledgment of personal debt (from the phrase "I owe you" or "I owe unto") that is generally limited to casual usage between acquaintances.A band that three members of Westlife were once inThe title of a song by the Canadian Indie Rock band Metric from the album Old World Underground, Where Are You Now?Illuminati University, a campaign setting for the GURPS roleplaying gameInvestor-Owned UtilityA cellular phone service provided by both Vodafone New Zealand and Vodafone Ireland. A customer can request an interim pay as you go credit of NZ$2 or €2 respectively, which is repaid when the credit is recharged.
See more at Wikipedia.org...

A paper having on it these letters, with a sum named, and duly signed; -- in use in England as an acknowledgment of a debt, and taken as evidence thereof, but not amounting to a promissory note; a due bill.