Definition - What does Pen Register mean?
A pen register is a device that reveals the phone numbers of outgoing calls when a phone is used. It does this by recording electronic impulses when telephones are in operation.
In the context of the law, pen registers can often be used in ongoing investigations to try to gain crucial information about who suspects are calling.
Justipedia explains Pen Register
There is some debate as to whether or not pen registers are a violation of the Fourth Amendment, which protects against unlawful searches and seizures. Different states have different legislation regarding when pen registers can be used lawfully. For example, many states require that law enforcement prove that a pen register is part of an ongoing investigation before they can legally use it without a warrant.
It is possible for pen registers to be applied through telephone company property. This helps to evade restrictions put forth in the Fourth Amendment, since technically, it is not the person's property that is being searched.