What term describes the amount of light falling on a unit of a digital sensor?

The term that specifically describes the amount of light falling on a unit of a digital sensor is exposure. Exposure refers to the overall amount of light that reaches the sensor during the time the shutter is open, which directly affects how bright or dark an image appears. It is a crucial component in photography and videography as it determines the final image quality; an image that is well-exposed has details in both the highlights and shadows.

While other terms in the options play significant roles in the exposure triangle—such as ISO, which refers to the sensitivity of the sensor to light; shutter speed, which dictates how long the sensor is exposed to light; and aperture, which controls the size of the lens opening—it is exposure that encompasses the total effect of all these elements on the sensor. Therefore, it is the correct term that most accurately addresses the overall impact of light intensity on the digital sensor.