Refers to the size of the imaging sensor. Common formats are 1/4″, 1/2″, 1/3″, 2/3″, and 1″.
The size of the sensor directly affects the field of view obtained. When using the same size lens on different format sensors, different viewing areas are obtained. For example, using a 6 mm lens on a 1/3″ sensor will give you a 37 degree field of view. Using the same lens on a 1/2″ sensor will increase the field of view to 56 degrees and 74 degrees on a 2/3″ sensor.
It is important to make sure you have a lens that was designed for at least the size of your sensor. For instance, you can use a 1″ format sensor on a 1/3″ camera, but you cannot use a 1/3″ lens on a 1″ camera.
If you use a lens designed for a smaller format camera, the image will be vignetted. Also, by using a lens designed for a larger format sensor, the field of view will be greater than what is specified.