A device or process which records or decodes routing.

A device or process which records or decodes routing, addressing, or signalling information transmitted by an tool or facility from which a wire or electronic announcement is transmitted, make available, however, that such information shall not include the contents of any communication, but such term does not include any device or process used by a provider or customer of a wire or electronic announcement service for billing, or recording as an incident to billing, for communications services provided by such provider or any device or process used by a provider or client of a wire communication service for cost accounting or other like purposes in the ordinary course of its commerce.

This is the current definition of a Pen Register, as amended by way of the 2001 USA PATRIOT Act. The original statutory definition of a pen register was created in 1984 as part of the Electronic Communications Privacy Act, which defined a "Pen Register" as:

A device which proceedings or decodes electronic or other impulses which identify the numbers called or otherwise transmitted on the telephone line to which such device is dedicated.