Invisible IR cameras are typically being referred to when speaking about CCTV security cameras that use 940nm IR LEDs instead of the more traditional 850nm IR LEDs.
So, what do these numbers mean? The 940nm and 850nm reference refers to the frequency of light emitted (measured in nano-meters) by the LED lights of the camera. 940nm LEDs are invisible to the human eye. 850nm LEDs are almost invisible, except that a red glow can be seen at the location of the LED itself.
For night-time covert surveillance, this is not ideal because the red glow gives away the location of the camera. There are both indoor and outdoor camera models available on the market that use 940nm infrared.