Main Entry: pen reg·is·ter
Function: noun
Etymology: 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.