1. A device which records or decodes electronic or other impulses which identify the numbers dialed or otherwise transmitted on the telephone line to which such device is attached, but such term does not include any device used by a provider, or customer of a wire or electronic communication service for billing, or recording as an incident to billing, for communications services provided by such provider or any device used by a provider, or customer of a wire communication service for cost accounting or other like purposes in the ordinary course of its business; see 18 USC §3127(3). (AR 381-10, US Army Intelligence Activities, 3 May 2007).
2. A pen register shows all outgoing phone numbers to a particular telephone has called.
3. [A device that] records or decodes dialing, routing addressing or signaling information transmitted by an instrument or facility from which a wire or electronic communication is transmitted, provided that such information must not include the contents of any communication. (FBI Domestic Investigations and Operations Guide, 16 Dec 2008).
Source: Terms & Definitions of Interest for DoD Counterintelligence Professionals, Office of the National Counterintelligence, https://www.dni.gov/files/NCSC/documents/ci/CI_Glossary.pdf