a device which records "numbers dialed or otherwise transmitted on the telephone line to which such device is attached

a device which records the number dialed on a telephone

a mechanical device attached to a given telephone line and usually installed at a central telephone facility

an electronic device which records

A device attached to a telephone line that maintains a record of each number dialed.

a device that records the telephone numbers of calls received by a particular telephone

An electronic surveillance device which attaches to a phone line and which registers every number dialed from a specific telephone. This surveillance device is not as effective as wire-tapping.

A pen register is an electronic device that records all numbers dialed from a particular telephone line. The term has come to include any device or program that performs similar functions to an original pen register, including programs monitoring Internet communications.