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The megapixel value is simply a shorthand means of expressing the camera's image resolution in a single figure.
Multiply the horizontal resolution by the vertical resolution to arrive at a value which expresses the total number of pixels that the image device can deliver.
Because this multiplication, typically of thousands times thousands, results in such a large value we use the term mega (indicating 10 to the power of six, which is 1,000,000) to express the result.
640 X 480 = 307,200 = 0.3 Mega pixels
Normally, resolutions less than 1 megapixel are not quoted in megapixels.
1280 X 1024 = 1.31 megapixels
This is pretty much the starting point for quoting megapixel values.
Obviously, the higher the value, the more pixels available, the better the image resolution, the more detail you will be able to resolve from the image.