listener /lisˈnər/ noun

A person who listens

listˈenership noun

The estimated number of listeners to a radio broadcast

listˈener-in noun (pl listˈeners-in).

listˈening-in noun

listening post noun

1. Orig a post where soldiers are stationed to hear what the enemy is doing
2. A position advantageous for the gathering of information about the affairs of another country, etc

listen in

1. To listen (to a radio broadcast)
2. To overhear intentionally a message intended for another person

listen up (informal)

To pay attention, listen hard