Depth of field
Mimicking a camera lens's focus: whatever lies in the focal plane is razor-sharp, the rest blurs softly.
A pinhole renderer keeps everything sharp at once, which looks unnaturally computer-generated. Real lenses only image a narrow distance sharply; points outside it spread into a small disc — the "circle of confusion" — whose size grows with distance from focus. The soft blobs of light this produces are called bokeh, an aesthetic in their own right.
In real time it is done in post: the depth buffer says how far each pixel lies, and a blur with a varying radius is applied accordingly, often split into a near and a far layer to stop sharp foreground from bleeding out. Games use it for focus pulls in cutscenes and to steer the eye. A cousin of motion blur, which instead smears across time.