Shot lacking sufficient light — blacks crush, detail is lost. Can be intentional for mood, but barely recoverable in grade.
You're shooting a scene and notice on the monitor: the sensor isn't getting enough photons. The shadows turn to mush, the midtones get crushed — that's underexposure. Not to be confused with a deliberate low-key aesthetic: underexposure is the result of too little available light for a given ISO, aperture, and shutter speed. You'll recognize it immediately on set: the image looks muddy, separation between objects is lost, and even in the darkest areas, you can't see any tonal values anymore — just digital noise or black holes.
The critical point: Underexposure cannot truly be salvaged in grading. You can push the curves, boost the shadows as much as you want — but you'll only bring noise and artifacts to the surface. You no longer have data worth lifting. Unlike overexposure, where you still have headroom in the highlights (if you shoot RAW), underexposure is a total data loss. The fine tonal gradations, the color separation — gone. This is particularly treacherous with skin tones and textures: they appear flat and unrealistic, no matter how much you tweak in DaVinci.
In practical workflow, this means: expose to the right — meaning, increase exposure until the histogram is just about to clip. This isn't aggressive or overexposed, but maximum data utilization. With digital cameras (especially Sony, RED, Alexa), the best detail information resides in the upper two-thirds of the sensor. It's better to expose slightly brighter and then filter in the edit, rather than struggling with a muddy shot later. It was different with traditional film cameras — you could use an underexposed style to create grain. Digitally, it only brings problems.
Deliberate underexposure — meaning stylistic darkness — is something else. When you're creating a noir-like scene with intentionally dark lighting, you still expose correctly for the highlights and let the shadows fall naturally. That's not underexposure, that's lighting design. The difference: with correct exposure, you still have tonal values and color in the dark areas. With true underexposure, you only have pixel mush.