What is a flat?

I seen in the photo section where several threads was referring to flats. Something about a bed sheet?

a 'flat' (flat field image) is an image taken to calibrate a digital picture. An image is taken of an evenly illuminated featureless object (such as a t-shirt, a wall, a sheet, a white card, a specially constructed box, etc.). This image records variations in sensitivity over the ccd frame--i.e. pixels that lie in the shadow of a dust particle will not record as much light as others; pixels on the edge of the frame will be dimmer due to vignetting. The flat field image records where all the various shadows and bright spots in the optical system are. Dividing an image by this 'flat field image' will brighten the dim parts, and dim the bright parts to even out the illumination and 'flatten' the backround. Here is a sample flat field image with vignetting and dust shadows (I'm past due for a good cleaning):

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