) A number in each iPhone that is used by developers to distribute new apps directly into the phone for testing purposes before they are available in the Apple App Store. The UDIDs are used to create an Apple-certified provisioning file that is dragged with the app into iTunes.
The UDID is not visible on the phone. It has to be revealed in iTunes, and it cannot be copied like ordinary text. The user has to take a screenshot of the UDID or hand copy all 40 hexadecimal digits into a document. There are also apps, such as UDID Sender, that extract the UDID and email it.
Perhaps No More
In 2011, Apple informed developers that it would be eliminating the iPhone UDID in the future, because the UDID can be associated with names, phone numbers and other user information. In September 2012, AntiSec hackers stole 12 million UDIDs from a BlueToad Inc. computer and uploaded about one million to the Internet, confirming the need to abandon the UDID in the iPhone.
Obtaining the UDID
When the iPhone is connected to the computer, the 40-digit hexadecimal UDID number can be revealed in iTunes by clicking the serial number.