UDID
Updated: 09/03/2019 by Computer Hope
Short for Unique Device Identification, a UDID is a sequence of numbers assigned to each smartphone or tablet running the iOS operating system. UDID is a proprietary system used by Apple to transfer information to and from iCloud and specific iPhone or iPad devices linked using the same Apple ID.
The UDID number of your iOS device is found in iTunes when it is plugged into a computer with a USB cable.