Effective Pixels
The number of photosites actually contributing to the final image, as distinct from the total on the chip.
Measured in
MP
In depth
Sensors carry extra rows and columns around their edges, some masked entirely to measure the black level and dark current, others used to give demosaicing algorithms neighbours at the frame boundary. The difference is typically a few per cent, which is why specifications quote both a total and an effective figure.
Examples in practice
A sensor listed at 26.2 total megapixels may deliver 24.2 effective megapixels in the final file.