Colour Filter Array
A mosaic of coloured filters over the photosites that allows a monochrome sensor to record colour.
In depth
Each photosite receives only one colour of light, so each records only one of the three values needed for a full-colour pixel. The missing two are estimated from neighbours during demosaicing. The array costs roughly two-thirds of the incoming light and some resolution, which is why monochrome-only cameras are both sharper and more sensitive.
Examples in practice
A dedicated monochrome camera without a filter array gains roughly a stop of sensitivity and visibly finer detail.