Chromatic aberration
Color channels drifting apart at the edges of a frame, leaving red and cyan fringes like a cheap lens.
See it
What it is
A lens bends short wavelengths harder than long ones, so blue and red never quite focus in the same place. The result is a thin colored fringe along high-contrast edges: red or magenta on one side, cyan or green on the other, getting worse toward the corners of the frame.
As an effect you fake it by scaling or offsetting the red and blue channels in opposite directions, with the amount growing radially from the center. That radial falloff is the tell that separates it from RGB split, where every channel slides the same distance across the whole frame. Games use it as a cheap 'this is a real camera' cue; cyberpunk and VHS looks crank it far past anything optical.
Gotcha: it is an edge-detection amplifier for eyestrain. A couple of pixels of fringe on a hero image is atmosphere, the same amount under body copy makes text feel wrong to read and can nauseate people. Keep it near zero at the center, cap it at the corners, and never put it on a layer that contains text you need read.
Ask AI for it
Add chromatic aberration to this image: shift the red channel outward and the blue channel inward along the vector from the frame center, with the offset scaling from 0px at the center up to about 4px at the corners, and leave green fixed. Composite the channels back together so high-contrast edges show thin red/cyan fringes. Keep the center sharp and subtle, no fringing on text.