listen in

verb3

1. •
   To listen without participating.

   • Do you mind if I listen in on your meeting tomorrow?
   • We invite you to listen in to our radio broadcast.
2. •
   (old-fashioned) To listen to a radio broadcast.

   • Did you listen in to the concert last night?
3. •
   To eavesdrop; to listen secretly.

   • You should close the door for conversations like that. You never know who might be listening in.