unique device identifier (UDID)

A unique device identifier (UDID) is a 40-character string assigned to certain Apple devices including the iPhone, iPad, and iPod Touch. Each UDID character is a numeral or a letter of the alphabet.

Using the UDID, third parties, particularly app vendors, can track subscriber behavior. These companies can learn what the user likes, and then tailor advertising accordingly. Unfortunately, this UDID feature carries the potential for abuse. In 2010, some users began to complain about unauthorized exposure of their individual identities. Some apps, for example, associated users' real names with device UDIDs without subscribers' knowledge or permission. 

Because of mounting privacy concerns on the part of device owners, Apple began discouraging app developers from tracking customers with UDIDs. In 2012, Apple began rejecting apps that make use of UDIDs.