Dynamic resolution
Also said as Dynamic resolution scaling, DRS, Resolution scaling
The game watches its own frame time and lowers the number of pixels it renders when things get heavy, then scales the picture up to fill the screen.
Players notice a stutter long before they notice a softer picture, so trading pixels for time is usually a good deal. It only helps if drawing pixels is what is slow; it can't fix an overloaded CPU.
Say it
“Hold 60 with dynamic resolution: keep GPU time under 15 ms by dropping as low as 70%, and recover slowly so the sharpness does not visibly pump.”
Where you’ve seen it
Doom (2016) changes its horizontal resolution on PS4 and Xbox One to protect 60 frames a second, as id Software confirmed. Its own anti-aliasing is built to hide the changes.
Not to be confused with upscaling
Upscaling, such as DLSS or FSR, is how a smaller image gets back to full size. Dynamic resolution is the decision about how small to render in the first place. They are often used together.