In digital display devices, this is a measure indicating the percentage of the available area that is used for active pixels. Some displays have obvious inactive 'frames' around individual pixels, leading to the description 'screen-door' effect when viewing such a display from insufficient distance.
The ratio between the transmissive portion of the pixel and its surrounding electronics, also known as fill factor. Generally, this is a limiting factor for luminance, the higher the aperture ratio; the brighter the luminance.