closed circuit television.
Security cameras. I bought a DVR (digital video recorder) from tiger direct I think. It came with 8 cameras that have infrared capability. (ability to see in the dark). You can view you cameras live and review video if you wish. It varies on cost. My DVR can hold 16 cameras and has 1 TB of memory. I think I spent around 300 for the set. I bought additional cameras to use as well that were upgrades from what the kit came with. There are many cameras you can get. You can get a fixed shot which is just a stationary camera or you can get a PTZ (pan/tilt/zoom) which allows you to control it remotely and move it around and zoom in and out. My DVR gives me the ability to view cameras when I switch my TV to VGA mode or I can access them via an app on my phone if I'm away from the house.
My son-in-law has the night vision camera to catch what type of wild critters come to the iron bars back fence that butts up against a wild preserve - he's caught Deer, Coyotes, Bobcats, Skunks, Raccoons, Oppossums, etc on film. The area is so infested with predators that the neighborhood has to bring in their pets at night. Camera is really worth the cost.