Underexposure
An exposure that records less light than the scene needs, producing dark, noisy shadows and muddy colour.
Measured in
stops (EV)
In depth
Because sensor noise is fixed and signal is not, underexposing hurts the shadows disproportionately: brightening a file in post amplifies the noise that was already there. A modest, deliberate underexposure to protect highlights is a valid strategy on cameras with strong shadow recovery, but two or three stops is rarely recoverable cleanly.
Examples in practice
A concert shot metered for the spotlight leaves the crowd two stops under; lifting them in post reveals blotchy colour noise.