Alpha blending
Laying a translucent pixel over what is already in the image, weighted by its alpha value.
The formula is out = src·α + dst·(1−α) — the "over" operator, formalized by Thomas Porter and Tom Duff in 1984. It is how glass, smoke, soft UI fades and antialiased silhouettes blend into the background. Blending happens in the blend stage at the end of the pipeline, after the fragment has been shaded, reading whatever is already sitting in the framebuffer.
The catch is order. Blending is not commutative, and the z-buffer cannot resolve several transparent surfaces stacked behind one another — it stores only one depth per pixel. So transparent objects have to be drawn after the opaque ones and sorted back-to-front. To avoid the sort there is alpha testing (a hard cutout) or order-independent transparency.