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pen register n
[perhaps from the original use of a pen to mark the dots or dashes used in counting the numbers dialed]
: a device that registers the numbers dialed from a telephone compare wiretap NOTE: A court order is always required for the use of a pen register in a criminal investigation, but such use has not been considered a search or interception of communication by the U.S. Supreme Court. Some states have disagreed and discern a privacy interest in such information.
Source: Merriam-Webster's Dictionary of Law ©1996. Merriam-Webster, Incorporated. Published under license with Merriam-Webster, Incorporated.