Exposing the film / image sensor to less light than is needed to render the scene as the eye sees it Results in a too dark image
Failure to expose correctly because not enough light has struck the film or sensor to faithfully render the color and brightness values Underexposed pictures are dark; the more the underexposure the darker they become Color also suffers when film is underexposed, although a slight amount of underexposure can be used to increase color saturation in certain color slide films