an exposure where the shutter is opened but no light is allowed to hit it so it only measures the energy from the CCD itself (dark current)

an image of these hot pixels that you can subtract from your target image to remove this noise

an image taken using the same settings used to image your target but with no light hitting the CCD

an image taken with the CCD's shutter closed

an image that has been shot with the telescope front cover on

an integration in which no light strikes the CCD

A dark frame is simply a CCD image taken with the camera's shutter closed. This image detects noise generated by dark current. An image of the same duration is taken with the shutter open, detecting both the noise and the object being imaged. Using image processing software to subtract the dark frame from the raw image leaves only the object. The dark noise is eliminated.

A dark frame is a photography technique to identify in a CCD imaging device by recording without exposing the CCD, usually by leaving the shutter closed. The noise is often caused by hot pixels. Hot pixels are sensors on the CCD with higher than normal charge leakage.