Integer scaling
Upscaling a low-resolution image by whole-number factors — 2×, 3×, … — so every source pixel becomes an identical square.
Scale a 320×240 image to an arbitrary screen size and some source pixels must cover more target pixels than others. The result is uneven "pixels" that shimmer and wreck the crisp grid pixel art depends on. Integer scaling refuses fractions: you pick the largest whole factor that fits and let the rest of the screen become a black border.
That is why emulators and pixel-art games often run at 4× or 5× rather than "fill the screen". The method is usually paired with nearest-neighbor filtering so no blur creeps in, and the unused area is handled with letterboxing or pillarboxing. Modern GPU drivers now expose their own "integer scaling" option, precisely because the desktop compositor's scaling would otherwise smear old pixel content.