Order-independent transparency
The umbrella term for techniques that render transparent surfaces correctly without first sorting them back-to-front.
Ordinary alpha blending is order-dependent: stacking layer on layer gives a result that depends on the order they are drawn, so transparent geometry normally has to be sorted back-to-front. That breaks down when surfaces intersect or when a single object overlaps itself — there is no correct sort order at all — and the sort itself costs time every frame.
OIT solves it per pixel. Depth peeling (Cass Everitt, 2001) peels off one transparent layer at a time. Per-pixel linked lists gather every fragment at a point and blend them in order afterwards, a modern take on Carpenter's A-buffer from 1984. Weighted blended OIT (McGuire and Bavoil, 2013) gives up exact ordering and weights contributions by depth instead — approximate but cheap. All of them rely on correct premultiplied alpha.