The image adjustment that allows users to change the overall lightness of an image is typically referred to as "Brightness" or "Exposure."
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Brightness adjustment modifies the overall lightness or darkness of the image, making it brighter or darker without altering the contrast significantly.
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Exposure adjustment increases or decreases the exposure of an image, which can also affect its lightness, usually by changing the sensitivity to light of the image.
Additionally, adjusting the Gamma can also affect the midtones of an image, influencing perceived lightness without changing the black and white points.
Different software may have slightly different names for these adjustments, but they generally serve similar functions in terms of lightness control.