What term describes the measure of how much light is let in by the camera lens?

Aperture is the term that describes the measure of how much light is let in by the camera lens. It refers to the opening in the lens through which light passes into the camera body. The size of this opening can be adjusted, allowing photographers to control the amount of light that reaches the camera's sensor. A wider aperture (indicated by a smaller f-number) allows more light to enter, resulting in brighter images, while a smaller aperture (indicated by a larger f-number) allows less light in, making images darker.

In addition to controlling light, aperture also plays a critical role in depth of field, influencing how much of the scene appears in focus. A wide aperture creates a shallow depth of field, which is often used to isolate subjects from their background, while a narrow aperture increases the depth of field, keeping more of the scene in focus.

The other terms—ISO, shutter speed, and exposure—are related to photography but do not specifically measure the light let in by the lens. ISO refers to the sensitivity of the camera sensor to light, shutter speed determines how long the sensor is exposed to light, and exposure is the overall measurement of light captured in a photograph, influenced by both aperture and shutter speed.