pen registerpen register
A monitoring device that records addressing or routing information, such as phone numbers or network addresses, about outgoing electronic communications from a particular source, but does not record the message contents.
Origin of pen registerProbably originally a term from electric telegraphy making reference to the pens or styluses that recorded messages on paper tape in the register or recording device, of the telegraph
pen register - Legal Definition
A device that records dialed telephone numbers. Pen registers have been deemed by federal authorities to be beyond the scope of wiretapping restrictions, because they do not intercept, hear, or monitor conversations. Some legislatures, however have ruled that their citizens are entitled to a higher degree of privacy.