Glow (outer glow / neon)
A soft colored halo spreading outward from an edge, with no offset, so the element looks like it is emitting light. Neon.
See it
What it is
A glow is a shadow with no offset: a soft, blurred halo in a bright color spreading outward from an element's edges, so the thing reads as emitting light rather than sitting under it. Photoshop calls it Outer Glow; on the web it is 'box-shadow' or 'text-shadow' with 0 offset and a generous blur, or 'filter: drop-shadow()' when the shape is irregular.
Reach for it for neon signage, cyberpunk and vaporwave interfaces, focus rings, and 'this element is active or charged' states. The trick that separates real neon from a fuzzy blob is stacking: a near-white core, then a tight saturated halo, then two or three progressively wider and fainter ones in the same hue. Real neon tubes also glow onto nearby surfaces, so tint the surrounding background slightly toward the glow color.
Gotcha: glow only reads as light against a dark background. On white it just looks like a colored smudge or a printing error. It is also close kin to bloom, but the distinction matters: glow is applied per element on purpose, while bloom is a post-process that bleeds from any bright pixel in the whole scene. And glowing text is a contrast trap, so keep the letterforms themselves at readable contrast and let the halo be decoration.
Ask AI for it
Give this heading and button a neon glow on a near-black background. Stack four text-shadows in the same hue with zero offset and increasing blur: 0 0 4px near-white, 0 0 10px full-saturation color, 0 0 30px the same color at 60% alpha, 0 0 70px at 30% alpha. Keep the letterform fill a very light tint of the hue, not pure saturated color, so it reads as the hot core of the tube. Add a faint radial gradient of the same color behind the element so the glow appears to spill onto the surface. On hover, brighten the inner two layers only.