Cat's-eye Bokeh
The lens-shaped, off-round rendering of out-of-focus highlights near the frame corners, caused by optical vignetting.
In depth
At the frame edges the barrel clips the cone of light from one side, so a circular highlight is cut into a shape resembling a cat's pupil, oriented toward the frame centre. Stopping down restores the circles because the iris, not the barrel, becomes the limiting aperture. In quantity across a busy background, the effect creates a swirling appearance prized in some vintage lenses.
Examples in practice
Backlit foliage behind a portrait at f/1.4 shows round highlights at the centre and elongated slivers in the corners.