What Does Underexposure Mean When Talking About Photography?

If too little light is recorded when capturing an image the image is said to be ‘underexposed’; The imaging medium i.e. film, or sensor has not been exposed to light for long enough to capture a sufficiently bright image. Underexposure of an entire image occurs when the combination of shutter speed, aperture and ISO chosen to capture an image have not been chosen correctly by the shooter, or metered correctly by the camera. Underexposure in dark areas of some scenes is unavoidable if the dynamic range being captured is too high, for example shooting a subject backlit by bright sunlight, but can be compensated for using techniques such ass fill flash or exposure stacking. Underexposure can also be corrected in post processing but is done so at the risk of introducing unwanted noise to an image.