White Level
The numerical value at which a sensor's photosites are considered saturated, defining the clipping point of a raw file.
Measured in
ADU
In depth
The white level is often slightly below the theoretical maximum for the bit depth, because manufacturers leave headroom for variation between photosites. Raw converters use it to identify which pixels have clipped, and knowing that some channels clip earlier than others is what makes partial highlight reconstruction possible.
Examples in practice
A raw converter reconstructs a slightly clipped highlight by inferring the red channel's value from the unclipped green and blue.