A Mobile Device ID, or Mobile Device Identifier, is a unique identifier which can be used to identify mobile devices.
The Mobile Device ID can usually only be accessed via an app and not from the mobile web. Since it allows them to track user information, advertisers use these identifiers for different mobile marketing issues. Using the mobile device identifier, they can determine if they have already served an ad to a specific user and to retarget and frequency cap them.
Mobile Device Identifiers have changed over time. Originally the Android ID or UDID (Universal Device ID for Apple devices) were used as device IDs. But due to privacy concerns Apple, Android and Windows have all moved to a format that gives users more control over their privacy through the use of a new advertising ID.
The device identification numbers should remain constant for the lifetime of the device unless a factory reset is performed.