ISO

ISO or what old-timers called ASA. DIN is another acronym for the same thing on a different scale.

ISO - International Organization of Standardization

In photographic terms, ISO refers the sensitivity of the sensor/pixels to light. The higher the ISO, the more sensitive to light for low light situations. Faster shutter speeds or smaller apertures can be used. With film cameras, increased ISO films were more grainy. A similar visual effect is seen in digital cameras, but is called noise. Newer digital cameras have built-in noise reduction filters. Post processing an image can reduce the noise level as well. It is still recommended to capture the image at the lowest possible ISO setting after determining the shutter speed and/or aperture settings.